Qbs: Avoid subdirs with ..

It is not really supported and has subtle issues, which result
qmake no longer finding the generated .qmake.cache for Qbs
(e.g. when Qt Creator is a subdir of another .pro file).

Change-Id: I7a4a1bb6bbde83966daeeccd68165981b6f90e03
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>