recognize QTQA_BUILD_PARTS

we are not using qt_parts.prf, so it needs manual handling.

Task-number: QTQAINFRA-607
Change-Id: Ieaf67d123db869af644e5f624140f6411275979f
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Simo Fält <simo.falt@digia.com>