Add g_source_set_dummy_callback()

Use g_source_set_closure() and g_close_set_meta_marshal() to allow
setting a do-nothing callback on any source.

https://bugzilla.gnome.org/show_bug.cgi?id=634239

gobject/gobject.symbols

@@ -128,6 +128,7 @@ g_value_get_flags
 g_io_channel_get_type
 g_io_condition_get_type
 g_source_set_closure
+g_source_set_dummy_callback
 #endif
 #endif