Add single-include guards to new headers

As pointed out by Christian Persch in bug 620173, all the new
gdbus and gsettings headers were missing these.

gio/gdbusproxy.h

@@ -20,6 +20,10 @@
  * Author: David Zeuthen <davidz@redhat.com>
  */
 
+#if !defined (__GIO_GIO_H_INSIDE__) && !defined (GIO_COMPILATION)
+#error "Only <gio/gio.h> can be included directly."
+#endif
+
 #ifndef __G_DBUS_PROXY_H__
 #define __G_DBUS_PROXY_H__