flatpak: Use URN for fontconfig DTD
It should not make any difference other than making it in sync with fontconfig https://gitlab.freedesktop.org/fontconfig/fontconfig/-/commit/9c46ef4aac87c42d013d0e7380b6aeb03e1a9949
It should not make any difference other than making it in sync with fontconfig https://gitlab.freedesktop.org/fontconfig/fontconfig/-/commit/9c46ef4aac87c42d013d0e7380b6aeb03e1a9949