[libc++] Make sure we use __ulock on Apple platforms

We forgot to include the header, which means that _LIBCPP_USE_ULOCK was
always undefined and we'd always use the fallback. Note that this doesn't
seem to fix #63737.

Differential Revision: https://reviews.llvm.org/D154718

libcxx/src/atomic.cpp

@@ -15,6 +15,8 @@
 #include <functional>
 #include <thread>
 
+#include "include/apple_availability.h"
+
 #ifdef __linux__
 
 #include <unistd.h>