[gnome-photos] Update to latest libgd

[Debarshi Ray <debarshir src gnome org>]

commit 337387e2385e72b3ece92e041968f51723b85a9c
Author: Debarshi Ray <debarshir gnome org>
Date:   Wed Aug 22 11:40:41 2018 +0200

    Update to latest libgd
    
    Silence some deprecation warnings with GLib 2.57.2 due to
    g_type_class_add_private.

 subprojects/libgd | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/subprojects/libgd b/subprojects/libgd
index ad398275..136cb912 160000
--- a/subprojects/libgd
+++ b/subprojects/libgd
@@ -1 +1 @@
-Subproject commit ad398275f3f64cd0e7d335a44ef048fcd1d8f4db
+Subproject commit 136cb9128aa1e0b31858feb8150ccd125d0a1432