Flatpak: use X11 access only if a X11 session is running

Requires flatpak 1.0 or newer

flatpak/org.gajim.Gajim.yaml

@@ -8,7 +8,7 @@ desktop-file-name-prefix: '(Nightly) '
 finish-args:
   - --share=ipc
   - --share=network
-  - --socket=x11
+  - --socket=fallback-x11
   - --socket=wayland
   - --socket=pulseaudio
   - --system-talk-name=org.freedesktop.GeoClue2