fix mac build missing includes

Summary:
mac build is failing with missing header includes, fix it so can tell if their are real problems

Test Plan:

CI.  Before, error on vector
```
CMakeFiles/tdigest_benchmark.dir/folly/stats/test/TDigestBenchmark.cpp.o -MF CMakeFiles/tdigest_benchmark.dir/folly/stats/test/TDigestBenchmark.cpp.o.d -o CMakeFiles/tdigest_benchmark.dir/folly/stats/test/TDigestBenchmark.cpp.o -c /Users/runner/work/folly/folly/folly/stats/test/TDigestBenchmark.cpp
In file included from /Users/runner/work/folly/folly/folly/stats/test/TDigestBenchmark.cpp:17:
/Users/runner/work/folly/folly/folly/stats/DigestBuilder.h:59:25: error: implicit instantiation of undefined template 'std::vector<double>'
    std::vector<double> buffer;
```

then error on array
```
CMakeFiles/constexpr_math_test.dir/folly/test/ConstexprMathTest.cpp.o -c /Users/runner/work/folly/folly/folly/test/ConstexprMathTest.cpp
/Users/runner/work/folly/folly/folly/test/ConstexprMathTest.cpp:38:29: error: implicit instantiation of undefined template 'std::array<unsigned long, 7>'
    std::array<res_t, size> res{};
```

After, build works

folly/stats/DigestBuilder.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <memory>
+#include <vector>
 
 #include <folly/Memory.h>
 #include <folly/SpinLock.h>

folly/test/ConstexprMathTest.cpp

@@ -16,6 +16,7 @@
 
 #include <folly/ConstexprMath.h>
 
+#include <array>
 #include <cmath>
 #include <limits>
 #include <type_traits>