Add org.freedesktop.Flatpak permission

Freeube can play videos using external players. This is need to launch
the external players from the sandbox by calling flatpak-spawn --host.
This isn't very ideal as it allows to run arbitrary commands from
inside the sandbox.

io.freetubeapp.FreeTube.yml

@@ -21,6 +21,7 @@ finish-args:
   - --talk-name=org.freedesktop.ScreenSaver
   - --talk-name=org.gnome.SessionManager
   - --talk-name=org.gnome.SettingsDaemon
+  - --talk-name=org.freedesktop.Flatpak
 modules:
   - name: freetube
     buildsystem: simple