[mutter] window: Fix a compiler warning

[Florian Müllner <fmuellner src gnome org>]

commit 4366687b951b56a74e803c0762dce9622e752516
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Aug 21 00:32:23 2013 +0200

    window: Fix a compiler warning

 src/core/window.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/core/window.c b/src/core/window.c
index b5a4984..5e7b3c9 100644
--- a/src/core/window.c
+++ b/src/core/window.c
@@ -5831,7 +5831,7 @@ meta_window_get_outer_rect (const MetaWindow *window,
 
       if (window->has_custom_frame_extents)
         {
-          GtkBorder *extents = &window->custom_frame_extents;
+          const GtkBorder *extents = &window->custom_frame_extents;
           rect->x += extents->left;
           rect->y += extents->top;
           rect->width -= extents->left + extents->right;