Fix another build problem introduced by the switch to QtQuick 2.

The project files enable the welcome screen for Qt >= 5.1, whereas the
source code assumes 5.0 is enough. As a result, the build fails for all
Qt versions 5.0.x.

Change-Id: I684e943c683ca5246c1637438b07a4ccacdf9b65
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>