[Qt] Ensure forwarding headers are generated before generating install rules

[Qt] Ensure forwarding headers are generated before generating install rules

The install rule for headers depends on qmake being able to walk the list of
headers in the $build_root/include/QtWebKit directory, but this directory is
empty until we've generated the forwarding headers. So we need to make sure
the forwarding headers are generated _before_ we run qmake on the api project
file. This applies to Qt 4.8 only, as Qt 5 will run synqt as part of the
root project file (due to the sync.profile file).

https://bugs.webkit.org/show_bug.cgi?id=71697

Reviewed by Simon Hausmann.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@99542 268f45cc-cd09-0410-ab3c-d52691b4dbfc