Do not iterate separately over all header files. This removes
duplicated header iteration code which already was out of sync.
As a side-effect, we do run the header check only on header files that
are considered part of the module, if moduleheaders is set in
sync.profile.

Change-Id: Iec4c8febe9128139d2f377a1da8ce08c24110a77
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>