fix non-git non-shadow non-prefix non-qtbase builds

these builds usually assume all headers in the qtbase build (== source
== install) dir, so the path for adding our pre-generated per-module
include paths needs to be triggered explicitly.

Change-Id: I57ec441d58cdf8186907ee6c36dce08daa206c49
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>