[gnome-control-center] Update submodule URL to current GitLab URL

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit ba81a788910ee060ae26d9a9248ba6a53e00cfc9
Author: Robert Ancell <robert ancell canonical com>
Date:   Fri Aug 24 11:48:36 2018 +1200

    Update submodule URL to current GitLab URL

 .gitmodules | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitmodules b/.gitmodules
index 220db1e88..5ab4a386d 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "subprojects/gvc"]
        path = subprojects/gvc
-       url = git://git.gnome.org/libgnome-volume-control
+       url = https://gitlab.gnome.org/GNOME/libgnome-volume-control.git