add RELAX NG .rng filename extension as XML
As described in: https://en.wikipedia.org/wiki/RELAX_NG
Used by GTK 4 for /usr/share/gtk-4.0/gtk4builder.rng
As described in: https://en.wikipedia.org/wiki/RELAX_NG
Used by GTK 4 for /usr/share/gtk-4.0/gtk4builder.rng