The documentation for both gui and widgets defines members of the
"helpsystem" group, which definition is given in qthelp.
Due to recent changes in QDoc that automatically link a class page to
its owning group, it is now required for those project to be dependent
on qthelp, so that it is possible to retrieve the necessary linking
information for the group definition.
Hence, such a dependency was added.
Change-Id: Id9399e76bbe10fb9ae0c05d637006b5cdc0b742b
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>