Target: Remove useless include

Add a include for QtDebug elsewhere where this include tended to add this
implicitly.

Also remove a using namespace directive that is unused and no longer works
since the namespace is no longer defined due to the removed include.

Change-Id: Ic216127a96c8aa11366defa0397f934d5a735925
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>