Don't expose internal functions in .so file

[zuiderkwast]

To avoid name collisions when linking projects with other versions of dict, sds, etc. we shouldn't expose these functions in the .so file.

We can compile with -fvisibility=hidden and explicitly add __attribute__((visibility("default"))) to the exported functions. It's explained in https://stackoverflow.com/a/4466766/975856.

(Related to #44.)