Move single-include guards inside include guards

gcc has optimizations for include guards that only work
if they are outermost in the the header.
https://bugzilla.gnome.org/show_bug.cgi?id=689810

glib/gqueue.h

@@ -24,13 +24,13 @@
  * GLib at ftp://ftp.gtk.org/pub/gtk/.
  */
 
+#ifndef __G_QUEUE_H__
+#define __G_QUEUE_H__
+
 #if !defined (__GLIB_H_INSIDE__) && !defined (GLIB_COMPILATION)
 #error "Only <glib.h> can be included directly."
 #endif
 
-#ifndef __G_QUEUE_H__
-#define __G_QUEUE_H__
-
 #include <glib/glist.h>
 
 G_BEGIN_DECLS