fix non-git non-prefix shadow builds

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>

mkspecs/features/qt_module_pris.prf

@@ -10,7 +10,7 @@
 #
 
 load(qt_build_paths)
-force_independent|!isEmpty(MODULE_FWD_INCLUDES): \
+force_independent|split_incpath: \
     CONFIG += need_fwd_pri
 mod_work_pfx = $$MODULE_QMAKE_OUTDIR/mkspecs/modules
 need_fwd_pri: \