make sure the vertical systary size hints are fixed

Summary:
on vertical panels the systray could get
stretched to a size taller than it should, getting
a lot of empty space
BUG:368146

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2645