Issue #26924: Fix Windows buildbots
sem_unlink is defined as
#define SEM_UNLINK(name) 0
under Windows.
diff --git a/Modules/_multiprocessing/multiprocessing.c b/Modules/_multiprocessing/multiprocessing.c
index f6938f9..d92a8bf 100644
--- a/Modules/_multiprocessing/multiprocessing.c
+++ b/Modules/_multiprocessing/multiprocessing.c
@@ -128,7 +128,7 @@
{"recv", multiprocessing_recv, METH_VARARGS, ""},
{"send", multiprocessing_send, METH_VARARGS, ""},
#endif
-#if defined(HAVE_SEM_UNLINK) && !defined(POSIX_SEMAPHORES_NOT_ENABLED) && !defined(__ANDROID__)
+#if !defined(POSIX_SEMAPHORES_NOT_ENABLED) && !defined(__ANDROID__)
{"sem_unlink", _PyMp_sem_unlink, METH_VARARGS, ""},
#endif
{NULL}