fontconfig: stop using xml:space; vital for nixos+2.11

Since fontconfig-2.11 the xml:space attribute makes it reject
/etc/fonts/fonts.conf, so it renders garbage and eats lots of CPU.

To use anything linked to fontconfig-2.11 you need to have this patch
applied to your running NixOS. That's why I'm pushing it to master
before the fontconfig update (as soon as I found and tested the fix).