Add explicit include for QVariant

This include is needed when building against Qt6 (by s/Qt5/Qt6, done for some ECM Qt6 compatibility experiments, no other plans here).

Not sure if it perhaps should be added to async.h instead, as QVariant::fromValue<T>(value) is used in the template method there in the template method Job::addToContext(const T &value) or if this still should be left to places which call the method and thus trigger a method instance.