turns out we need forwarding .pris in this case: without them,
QT_MODULE_INCLUDE_BASE points into the build dir, so we fail to find the
pre-generated headers.

an alternative would be writing primary module .pris which already take
that into account, but that would just add even more arcane code paths.

Task-number: QTBUG-51521
Change-Id: I59f2a0d3f2095c9dfa0a8d1cabfc007a30bd2d23
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>