Use glib/gi18n.h instead of libintl.h, as is done everywhere else.
In addition to following the common style, this allows building against
a patched glib on an embedded uclibc system

--- gtk/gtktrayicon-x11.c.orig	2008-06-10 11:15:21 +0000
+++ gtk/gtktrayicon-x11.c	2008-06-10 11:15:32 +0000
@@ -24,7 +24,7 @@
 
 #include <config.h>
 #include <string.h>
-#include <libintl.h>
+#include <glib/gi18n.h>
 
 #include "gtkintl.h"
 #include "gtkprivate.h"
--- tests/buildertest.c.orig	2008-06-10 13:25:32 +0000
+++ tests/buildertest.c	2008-06-10 13:26:12 +0000
@@ -20,7 +20,7 @@
  */
 
 #include <string.h>
-#include <libintl.h>
+#include <glib/gi18n.h>
 #include <locale.h>
 #include <math.h>