Fix compilation with Qt 5 / nmake / shadow builds

qmake/qt4 included the source directory automatically for most plugins,
which got dropped in qt 5 (change 0f548b1744e2). Make sure it's
included again ...

Change-Id: Ief06cf9b3c7afd8432df7511df5f917023d477ef
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>