gh-110014: Fix inconsistent struct definition in pycore_semaphore.h

The pycore_semaphore.h header is included by Python/lock.c and
Python/parking_lot.c. The macro `_POSIX_SEMAPHORES` was not consistently
defined across the two files (due to a missing include of `<unistd.h>`)
leading to different struct definitions. The RHEL8 ppc64le LTO buildbot
correctly warned due to this issue.

Author: colesbury

Include/internal/pycore_semaphore.h

@@ -20,6 +20,10 @@
 #   error "Require native threads. See https://bugs.python.org/issue31370"
 #endif
 
+#ifndef MS_WINDOWS
+#   include <unistd.h>              // _POSIX_SEMAPHORES
+#endif
+
 #if (defined(_POSIX_SEMAPHORES) && (_POSIX_SEMAPHORES+0) != -1 && \
         defined(HAVE_SEM_TIMEDWAIT))
 #   define _Py_USE_SEMAPHORES