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.c

@@ -3184,7 +3184,7 @@ meta_window_is_fullscreen (MetaWindow *window)
   return window->fullscreen;
 }
 
-static void
+void
 meta_window_tile (MetaWindow *window)
 {
   MetaMaximizeFlags directions;