build: Introspect some more types

While MetaStage, MetaWindowGroup and MetaDBusDisplayConfigSkeleton don't
appear explicitly in the public API, their gtypes are still exposed via
meta_get_stage_for_screen(), meta_get_*window_group_for_screen() and
MetaMonitorManager's parent type. Newer versions of gjs will warn about
undefined properties if it encounters a gtype without introspection
information, so expose those types to shut up the warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=781471

src/backends/meta-cursor-renderer.c

@@ -33,7 +33,7 @@
 #include <cogl/cogl.h>
 #include <clutter/clutter.h>
 
-#include "meta-stage.h"
+#include "meta-stage-private.h"
 
 struct _MetaCursorRendererPrivate
 {