Gtk+3: Fix cups related error in configure

This patch will disable cups for gtk+3.
The issue could be resolved by adding cups to the DEPENDS,
however in gtk+2 cups is also disabled, so fixing it here in the same way seems appropriate.

Error message is:
| checking for cups/cups.h... no
| configure: error: *** Sorry, cups-config present but cups/cups.h missing.

Signed-off-by: Samuel Stirtzel <s.stirtzel@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>