xmlto: Fix eval with w3m.graphicsSupport.

Having `nixpkgs.config.w3m.graphicsSupport = true;` leads to an infinite
recursion, because xmlto needs w3m for _text_ conversion and w3m with
graphics support needs imlib2, which in turn needs xmlto.

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