Flatpak: Explicitly set gui for gtk3.22 flatpak
gtk3.22-client flatpak .yml does not define gui to use, but relies on gtk3.22 being the default. Make it explicit, to avoid accidents in the future.
gtk3.22-client flatpak .yml does not define gui to use, but relies on gtk3.22 being the default. Make it explicit, to avoid accidents in the future.