[gnome-documents] build: Update to latest libgd

[Debarshi Ray <debarshir src gnome org>]

commit 42b93229b873fee5056376dcd93936a508dca182
Author: Debarshi Ray <debarshir gnome org>
Date:   Wed Nov 4 14:23:31 2015 +0100

    build: Update to latest libgd
    
    https://bugzilla.gnome.org/show_bug.cgi?id=757590

 libgd |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/libgd b/libgd
index bf0b5c9..f2b8fb3 160000
--- a/libgd
+++ b/libgd
@@ -1 +1 @@
-Subproject commit bf0b5c96a6b2f4b039fc12c2cef7729fc7cb6ecf
+Subproject commit f2b8fb3acbee7568f7cb864dd58da42c06412cb3