Workaround for broken recursive qmake call

Running qmake recursively does not take .depends into account and goes
through the SUBDIRS in the order they were added. Because of this, the
qmake_extras.gypi is generated in wrong order and is not used by ninja
later on. This causes cross compilation to fail.

Task-number: QTBUG-45706
Change-Id: I476930233824d5414e35151e6b9a2695abb4e9dd
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>