Use externs in common.h

[synthead]

This PR fixes an issue when compiling with gcc 10. extern statements are required when declaring variables in header files, so this adds these.

Without this PR, this is what is seen when compiling with gcc 10:

linking shared-object rpi_gpio/rpi_gpio.so
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:37: multiple definition of `gpio_mode'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:37: first defined here
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:43: multiple definition of `rpiinfo'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:43: first defined here
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:41: multiple definition of `pin_to_gpio'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:41: first defined here
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:45: multiple definition of `module_setup'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:45: first defined here
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:44: multiple definition of `setup_error'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:44: first defined here
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:42: multiple definition of `gpio_direction'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:42: first defined here
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:40: multiple definition of `pin_to_gpio_rev3'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:40: first defined here
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:39: multiple definition of `pin_to_gpio_rev2'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:39: first defined here
/usr/bin/ld: rb_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:38: multiple definition of `pin_to_gpio_rev1'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:38: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:42: multiple definition of `gpio_direction'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:42: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:45: multiple definition of `module_setup'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:45: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:44: multiple definition of `setup_error'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:44: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:43: multiple definition of `rpiinfo'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:43: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:41: multiple definition of `pin_to_gpio'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:41: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:40: multiple definition of `pin_to_gpio_rev3'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:40: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:39: multiple definition of `pin_to_gpio_rev2'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:39: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:38: multiple definition of `pin_to_gpio_rev1'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:38: first defined here
/usr/bin/ld: rb_pwm.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:37: multiple definition of `gpio_mode'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:37: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:45: multiple definition of `module_setup'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:45: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:44: multiple definition of `setup_error'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:44: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:43: multiple definition of `rpiinfo'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:43: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:42: multiple definition of `gpio_direction'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:42: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:41: multiple definition of `pin_to_gpio'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:41: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:40: multiple definition of `pin_to_gpio_rev3'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:40: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:39: multiple definition of `pin_to_gpio_rev2'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:39: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:38: multiple definition of `pin_to_gpio_rev1'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:38: first defined here
/usr/bin/ld: rpi_gpio.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:37: multiple definition of `gpio_mode'; common.o:/home/deploy/on-air/.misc/rpi_gpio/tmp/armv7l-linux-eabihf/rpi_gpio/3.0.2/../../../../ext/rpi_gpio/common.h:37: first defined here
collect2: error: ld returned 1 exit status
make: *** [Makefile:262: rpi_gpio.so] Error 1