Fix regression in mutter-wayland translations

Switching meta/util.h to gi18n.h was wrong, mutter is a library
and needs gi18n-lib.h, but that cannot be included from a public
header (since it depends on config.h or command line options),
so split util.h into a public and a private part.

https://bugzilla.gnome.org/show_bug.cgi?id=707897

src/core/main.c

@@ -48,7 +48,7 @@
 
 #include <config.h>
 #include <meta/main.h>
-#include <meta/util.h>
+#include "util-private.h"
 #include "display-private.h"
 #include <meta/errors.h>
 #include "ui.h"