Convert from old connect syntax to new one

Summary:
Starting plasmashell with callgrind I noticed a line:
QObject::connect: No such slot KIO::ListJob::slotTotalSize(KIO::filesize_t) in ..../kdesrc/frameworks/kio/src/core/listjob.cpp:289

Test Plan: start plasmashell. The warning line is gone.

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: broulik

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10537