QbsProjectManager: Fix building of single files.

After doing this once, the active file tags were not reset, so
building everything was no longer possible.

Change-Id: I98df5f55e129cc4a72921aff51201e1860ba939a
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>