[gnome-documents] build: Update to latest libgd

[Debarshi Ray <debarshir src gnome org>]

commit 4beaf400fbdb0bb8de3024c332fae1a7ea6465d8
Author: Debarshi Ray <debarshir gnome org>
Date:   Thu Feb 1 20:17:36 2018 +0100

    build: Update to latest libgd
    
    GdTwoLinesRenderer now respects its vertical alignment. As a result
    the title and author are now centred in the list view.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=723843

 subprojects/libgd |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/subprojects/libgd b/subprojects/libgd
index 4e0f712..9117650 160000
--- a/subprojects/libgd
+++ b/subprojects/libgd
@@ -1 +1 @@
-Subproject commit 4e0f712f570716886ebb6983e6fae5411d82d30c
+Subproject commit 9117650bda7f981ccb41adce8018bbbe45cdb3b3