native/posix: Don't include sys/types.h in sem

`sys/types.h` contains the native definition for `pthread_*_t`. This
causes clashes if you want to use `semaphore` and `pthread` in the same
application.
Merge pull request #806 from Kijewski/sem-pthread-native

native/posix: Don't include sys/types.h in semaphore.h