Adapt to removal of Mutter namespace

MutterWindow and MutterPlugin have been renamed to MetaWindowActor
and MetaPlugin, mutter_plugin_list_windows() to
meta_plugin_list_window_actors(). Adapt to those changes.

https://bugzilla.gnome.org/show_bug.cgi?id=632500

src/shell-global-private.h

@@ -7,7 +7,7 @@
 #include <gjs/gjs.h>
 
 void _shell_global_set_plugin      (ShellGlobal  *global,
-                                    MutterPlugin *plugin);
+                                    MetaPlugin   *plugin);
 void _shell_global_set_gjs_context (ShellGlobal  *global,
                                     GjsContext   *context);