qt5declarative: fix examples compile without qt widgets module

Fixes:

  .../qt5declarative-5.8.0/examples/quick/demos/photoviewer/photoviewer.pro
  Project ERROR: Unknown module(s) in QT: widgets

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>