modules/services/printing/cupsd.nix: tell cups to use 'gstoraster' instead of 'pstoraster'

The CUPS filter has been renamed in GNU Ghostscript 9.x. Fortunately,
Ghostscript ships with a MIME conversions file that informs CUPS about this,
so linking that file into /etc/cups works fine. I'm not sure whether it's a
particularly elegant solution, though.

svn path=/nixos/trunk/; revision=31489