nixos/fonts: Add unifont to list of default fonts.

This fixes #10077 because after some debugging it turns out that by
default we don't have a font which is able to display Chinese symbols.

Thanks to @anderspapitto, @kmicu and hyper_ch on IRC to help debugging
this issue, see log at:

http://nixos.org/irc/logs/log.20150926 starting at 19:46

With unifont we have a reasonable fallback font to ensure that every
written language is rendered correctly and thus less surprise for new
users who keep their font settings at the default.

Reported-by: Anders Papitto <anderspapitto@gmail.com>
Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/config/fonts/fonts.nix

@@ -31,6 +31,7 @@ with lib;
         pkgs.xorg.fontbh100dpi
         pkgs.xorg.fontmiscmisc
         pkgs.xorg.fontcursormisc
+        pkgs.unifont
       ];
 
   };