Add compatibility with GTK+ 2.18

In order to replace calls to deprecated GDK code, the current
development version of GTK+ is required. Add some basic compatibility
code to allow building mutter with GTK+ 2.18.

https://bugzilla.gnome.org/show_bug.cgi?id=587991

src/ui/frames.c

@@ -36,6 +36,7 @@
 #include "ui.h"
 
 #include "gtk-compat.h"
+#include "gdk-compat.h"
 
 #ifdef HAVE_SHAPE
 #include <X11/extensions/shape.h>