KJob::kill(...): use fully-qualified name for type of slot args

I searched for any string-based connects to that slot using https://lxr.kde.org/search?%21v=kf5-qt5&_filestring=&_string=SLOT%5C%28kill&_casesensitive=1 but those hits all relied on the default argument value, so not needing to deal with the incomplete type. So seems no follow-up work is needed