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/ui.c

@@ -32,6 +32,7 @@
 #include "theme.h"
 
 #include "inlinepixbufs.h"
+#include "gdk-compat.h"
 
 #include <string.h>
 #include <stdlib.h>