[gnome-documents] build: Update to latest libgd

[Debarshi Ray <debarshir src gnome org>]

commit b06a8689e0059839cf0e25ec37276d5be8da8f1f
Author: Debarshi Ray <debarshir gnome org>
Date:   Thu Feb 1 18:54:24 2018 +0100

    build: Update to latest libgd
    
    The GdTaggedEntry tests are now built when using Meson, and the
    GdTwoLinesRenderer respects the GTK_CELL_RENDERER_SELECTED_STATE.

 subprojects/libgd |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/subprojects/libgd b/subprojects/libgd
index 234f224..4e0f712 160000
--- a/subprojects/libgd
+++ b/subprojects/libgd
@@ -1 +1 @@
-Subproject commit 234f224d8cef69a1fc6dc37059cbb31fc835c4c1
+Subproject commit 4e0f712f570716886ebb6983e6fae5411d82d30c