darwin,test: include AvailabilityMacros.h

It's necessary for MAC_OS_X_VERSION_10_12 to be defined.

Refs: https://github.com/libuv/libuv/issues/2491
PR-URL: https://github.com/libuv/libuv/pull/2516
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/test-fs-event.c

@@ -25,6 +25,10 @@
 #include <string.h>
 #include <fcntl.h>
 
+#if defined(__APPLE__) && !TARGET_OS_IPHONE
+# include <AvailabilityMacros.h>
+#endif
+
 #ifndef HAVE_KQUEUE
 # if defined(__APPLE__) ||                                                    \
      defined(__DragonFly__) ||                                                \