[gnome-music/wip/mschraal/flatpak-gitlab-migration] build: Update libgd to GNOME GitLab url

[Marinus Schraal <mschraal src gnome org>]

commit fc9f424bbbd8820ef918ca910ea9129512a1c8a7
Author: Marinus Schraal <mschraal gnome org>
Date:   Sun May 27 17:59:24 2018 +0200

    build: Update libgd to GNOME GitLab url

 .gitmodules | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitmodules b/.gitmodules
index 054b71cc..fa136361 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "subprojects/libgd"]
        path = subprojects/libgd
-       url = https://git.gnome.org/browse/libgd
+       url = https://gitlab.gnome.org/GNOME/libgd.git