Fix build for macOS 11 SDK

This file uses MAC_OS_X_VERSION_*, which are defined in
AvailabilityMacros.h

Test: build with macOS 11 SDK
Change-Id: If60aeb0e2eb0891b448fbba5db5d03079d826967
diff --git a/Modules/getpath.c b/Modules/getpath.c
index 27f3381..181354d 100644
--- a/Modules/getpath.c
+++ b/Modules/getpath.c
@@ -7,6 +7,7 @@
 #include <string.h>
 
 #ifdef __APPLE__
+#include <AvailabilityMacros.h>
 #include <mach-o/dyld.h>
 #endif