Restore the condition for deciding what a private header is from
fe383e04 to not check *_pch.h files and third party headers.
This amends commit eddb938c.
Change-Id: Idc55269478e7f2c98fb7debceef8a53c3cb5c657
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>