[gnome-documents] build: Update to latest libgd

[Debarshi Ray <debarshir src gnome org>]

commit caa089ed5fa89316197e51943edcec092b37b103
Author: Debarshi Ray <debarshir gnome org>
Date:   Sat Oct 6 19:28:39 2018 +0200

    build: Update to latest libgd
    
    Silence some deprecation warnings due to g_type_class_add_private and
    gtk_style_context_get_border_color.

 subprojects/libgd | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/subprojects/libgd b/subprojects/libgd
index 9117650b..039eff81 160000
--- a/subprojects/libgd
+++ b/subprojects/libgd
@@ -1 +1 @@
-Subproject commit 9117650bda7f981ccb41adce8018bbbe45cdb3b3
+Subproject commit 039eff81546b1076bcad5eab5ee6bc37e702e0a5