[gnome-games/retro-gtk-0-18] flatpak: Ensure we use retro-gtk 0.18

[Adrien Plazas <aplazas src gnome org>]

commit 3c0088f8d7d41122d4eeb10651555aea7ce833ab
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 | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/flatpak/org.gnome.Games.json b/flatpak/org.gnome.Games.json
index 83ebd0cf..e0cd0722 100644
--- a/flatpak/org.gnome.Games.json
+++ b/flatpak/org.gnome.Games.json
@@ -218,6 +218,7 @@
             "sources" : [
                 {
                     "type" : "git",
+                    "branch" : "retro-gtk-0-18",
                     "url" : "https://gitlab.gnome.org/GNOME/retro-gtk.git";
                 }
             ]