WorkerScript is no longer part of QtQml, but in its own module.

Change-Id: Id0df8a1321053092d4ffe005c371b23e4b5ed88a
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Nico Vertriest <nico.vertriest@qt.io>