Add private _g_bus_get_singleton_if_exists() function

This is used by g_test_dbus_down() to ensure the GDBusConnection
gets disposed, but not create one if the singleton already got
disposed.

https://bugzilla.gnome.org/show_bug.cgi?id=672985

gio/gdbusprivate.h

@@ -143,6 +143,9 @@ void _g_dbus_object_proxy_add_interface (GDBusObjectProxy *proxy,
 void _g_dbus_object_proxy_remove_interface (GDBusObjectProxy *proxy,
                                             const gchar      *interface_name);
 
+/* Implemented in gdbusconnection.c */
+GDBusConnection *_g_bus_get_singleton_if_exists (GBusType bus_type);
+
 G_END_DECLS
 
 #endif /* __G_DBUS_PRIVATE_H__ */