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/core.c

@@ -29,6 +29,7 @@
 #include "workspace-private.h"
 #include <meta/prefs.h>
 #include <meta/errors.h>
+#include "util-private.h"
 
 /* Looks up the MetaWindow representing the frame of the given X window.
  * Used as a helper function by a bunch of the functions below.