nixos/cupsd: Ignore collisions in cups-progs.

There currently are collisions between the main CUPS package and the
filters package, which are:

  * $storepath/share/cups/banners/classified
  * $storepath/share/cups/banners/confidential
  * $storepath/share/cups/banners/secret
  * $storepath/share/cups/banners/standard
  * $storepath/share/cups/banners/topsecret
  * $storepath/share/cups/banners/unclassified
  * $storepath/share/cups/data/testprint

And they actually have different content, so let's ignore those for now
until we have a better fix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>