automate handling of generated headers some more

let the syncqt + qt_module_header.prf pair handle generation of
forwarding headers.

in qtbase this is ineffective to some degree, as the need to create
QtCore's forwarding headers early for QtBootstrap requires qtbase.pro
already doing the real work, but at least we get the verification that
nothing breaks.
Other Modules (TM) will need the full functionality.

Change-Id: Ifd3dfa05c4c8a91698a365160edb6dabc84e553f
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>