[gnome-boxes] Update libgd

[Felipe Borges <felipeborges src gnome org>]

commit 8d40acd2c74001e884373b2e49ba35ae9d43ab8c
Author: Felipe Borges <felipeborges gnome org>
Date:   Tue May 2 12:44:59 2017 +0200

    Update libgd
    
    We can benefit from the fixes to the selection-mode
    and drag and drop counter on HiDPI.

 libgd |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/libgd b/libgd
index 752f65e..63d1488 160000
--- a/libgd
+++ b/libgd
@@ -1 +1 @@
-Subproject commit 752f65e91ea0d9a2ee8a2d21343bbd97bd0d038a
+Subproject commit 63d1488b3bcea686e85dd7273365861eacbe8442