[gnome-boxes] Update to latest libgd

[Zeeshan Ali Khattak <zeeshanak src gnome org>]

commit a6f2acbc08fd5786939bfac94d7ef67a63fbe606
Author: Zeeshan Ali (Khattak) <zeeshanak gnome org>
Date:   Thu Nov 8 11:08:14 2012 +0100

    Update to latest libgd
    
    This is to get the fix for bug#687286.

 libgd |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/libgd b/libgd
index 43134a3..6318d52 160000
--- a/libgd
+++ b/libgd
@@ -1 +1 @@
-Subproject commit 43134a3534f0468a7de992e11e236989a7c86357
+Subproject commit 6318d529754dbb2a5f95d3a546340f476b37bcc4