[aurorae] Fix visibility of Context Help button

Summary:
providesContextHelp is a property of DecoratedClient, not Decoration.

BUG: 413145
FIXED-IN: 5.17.1

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24851