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
1 job for wip/jtojnar/fontconfig-dtd in 41 minutes and 49 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Test
passed flatpak #1663012
flatpak

00:41:49