Don't depend on QtQuick plugin in QtWidgets

The accessible plugins directory contains a QtQuick plugin
as well as a widgets plugin. We need to specify the correct
one to avoid all widgets applications implicitly depending
on QtQuick on Android.

Change-Id: I72b2ee4fcd2d7914fc2ed7996f504ce2df79842d
Reviewed-by: Christian Stromme <christian.stromme@digia.com>