libgnomecups: Add CUPS >= 1.6 compatibility patch.

Patch is from Gentoo at:

http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/net-print/libgnomecups/files/libgnomecups-0.2.3-cups-1.6.patch

Fixes the following Hydra build failure:

http://hydra.cryp.to/build/250133/nixlog/5/raw

Thanks to @peti for the notification.

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