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: I0967ccc770888e07325118b51883b9a3c1358192
diff --git a/Modules/getpath.c b/Modules/getpath.c
index b727f66..5ce751a 100644
--- a/Modules/getpath.c
+++ b/Modules/getpath.c
@@ -11,6 +11,7 @@
 #include <string.h>
 
 #ifdef __APPLE__
+#  include <AvailabilityMacros.h>
 #  include <mach-o/dyld.h>
 #endif