Pin qtbase version so we can integrate QPA API changes

Will be unpined along with an update that matches the changes in
https://codereview.qt-project.org/#change,66591.

Change-Id: I84fc8b6d28c05d75fe199044a9a5c7cf0070518f
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>