[gnome-games] flatpak: Ensure we use retro-gtk 0.18

[Alexander Mikhaylenko <alexm src gnome org>]

commit b11c16ebccd06d30f7ed6a1297181d2b74041acb
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Sat Jan 18 23:31:13 2020 +0100

    flatpak: Ensure we use retro-gtk 0.18
    
    This is needed as retro-gtk will soon break its API on master.

 flatpak/org.gnome.Games.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/flatpak/org.gnome.Games.json b/flatpak/org.gnome.Games.json
index bd5986a5..786cdb13 100644
--- a/flatpak/org.gnome.Games.json
+++ b/flatpak/org.gnome.Games.json
@@ -219,7 +219,8 @@
             "sources" : [
                 {
                     "type" : "git",
-                    "url" : "https://gitlab.gnome.org/GNOME/retro-gtk.git";
+                    "url" : "https://gitlab.gnome.org/GNOME/retro-gtk.git";,
+                    "branch" : "retro-gtk-0-18"
                 }
             ]
         },