Run syncqt before tarring up the sources

Run syncqt to generate forward-include headers needed for
non-git builds.

The --skip-syncqt option is provided for backwards compatibility
with Qt 5.0.x versions and/or other use cases.

Change-Id: I57e976f7b1a0f95084a8a7d2ed20a1aa2b322d2b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>