window: Make meta_window_tile() semi-public

The previous tiling state of a grabbed window should be restored
if the drag operation is cancelled (by hitting the Escape key).
This might involve to meta_window_tile(), so export the function
in window-private.h.

https://bugzilla.gnome.org/show_bug.cgi?id=639988

src/core/window-private.h

@@ -438,6 +438,7 @@ void        meta_window_unmanage           (MetaWindow  *window,
 void        meta_window_calc_showing       (MetaWindow  *window);
 void        meta_window_queue              (MetaWindow  *window,
                                             guint queuebits);
+void        meta_window_tile               (MetaWindow        *window);
 void        meta_window_maximize_internal  (MetaWindow        *window,
                                             MetaMaximizeFlags  directions,
                                             MetaRectangle     *saved_rect);