Add missing doxygen for pthread funs and structs

sys/posix/pthread/include/pthread.h

sys/posix/pthread/include/pthread_attr.h

sys/posix/pthread/include/pthread_barrier.h

sys/posix/pthread/include/pthread_cancellation.h

sys/posix/pthread/include/pthread_cleanup.h

sys/posix/pthread/include/pthread_cond.h

sys/posix/pthread/include/pthread_mutex.h

sys/posix/pthread/include/pthread_mutex_attr.h

sys/posix/pthread/include/pthread_once.h

sys/posix/pthread/include/pthread_rwlock.h

sys/posix/pthread/include/pthread_scheduling.h

sys/posix/pthread/include/pthread_spin.h

sys/posix/pthread/include/pthread_threading.h

sys/posix/pthread/include/pthread_threading_attr.h

sys/posix/pthread/include/pthreadtypes.h

sys/posix/pthread/pthread.c

sys/posix/pthread/pthread_attr.c

sys/posix/pthread/pthread_barrier.c

sys/posix/pthread/pthread_barrierattr.c

sys/posix/pthread/pthread_mutex.c