Add missing file.

objc/objc-visibility.h

@@ -0,0 +1,9 @@
+#if defined _WIN32 || defined __CYGWIN__
+#	ifdef __OBJC_RUNTIME_INTERNAL__
+#		define PUBLIC __attribute__((dllexport))
+#	else
+#		define PUBLIC __attribute__((dllimport))
+#	endif
+#else
+#	define PUBLIC
+#endif