network-manager: Don't build Qt example programs

The purpose of "--enable-qt" option is not to build NetworkManager with
Qt support. It's name can be a bit confusing. The real purpose of this
option is to build Qt example programs, so we disable it as we also
disable the tests and documentation in the target.

Fixes:
   http://autobuild.buildroot.net/results/3b6/3b6a40c1683d0859a934e4d79e2048e97b193e94/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>