Qbs: Finish qbs evaluation future watcher

Looks like you need to do that after canceling it... This stops
"Evaluation" progress bars from sticking around.

Change-Id: Ie0a9f88ba96cba261939c210fddb11bc43779416
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>