Add compatibility with GTK+ 2.18

To replace all calls to deprecated code, GTK+ 2.20 is required - add
some basic compatibility code, so that it is still possible to build
mutter with GTK+ 2.18 when not using -DGSEAL_ENABLE.

https://bugzilla.gnome.org/show_bug.cgi?id=595496

src/ui/frames.c

@@ -35,6 +35,8 @@
 #include "prefs.h"
 #include "ui.h"
 
+#include "gtk-compat.h"
+
 #ifdef HAVE_SHAPE
 #include <X11/extensions/shape.h>
 #endif