[gnome-ostree-integration] Update libgsystem's submodule

[Emmanuele Bassi <ebassi src gnome org>]

commit 3a7cb39a3675b3176d24fbb786f929a4d5eb9d37
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Mon Mar 18 11:51:29 2019 +0000

    Update libgsystem's submodule
    
    The repo was migrated to Archive on GitLab.

 .gitmodules | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitmodules b/.gitmodules
index f456a14..4334581 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "src/libgsystem"]
        path = src/libgsystem
-       url = git://git.gnome.org/libgsystem
+       url = https://gitlab.gnome.org/Archive/libgsystem.git