flatpak: Prevent X11 access in a Wayland session with newer Flatpak
The unstable Flatpak 0.11.1 release added a fallback-x11
socket that doesn't expose the XWayland socket inside a Wayland session if the application can run as a native Wayland client. However, if the application is being run by an older Flatpak, then this socket type is silently ignored, so it doesn't break compatibility.