notify: fix macOS build, don't include <atomic> if we don't have it

dtool/src/prc/notify.cxx

@@ -19,9 +19,12 @@
 #include "filename.h"
 #include "config_prc.h"
 
-#include <atomic>
 #include <ctype.h>
 
+#ifdef PHAVE_ATOMIC
+#include <atomic>
+#endif
+
 #ifdef BUILD_IPHONE
 #include <fcntl.h>
 #endif