Fix no such signal error on assistant startup with no webkit.

QObject::connect: No such signal HelpViewer::printRequested() in
qttools/src/assistant/assistant/centralwidget.cpp:626

Change-Id: If7dee3b8d38f659ca037b9d4ed73e01779f07c67
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>