use plasma icon as icon for plasma notification category

Summary:
kde is the kde logo, start-here-kde is our app menu icon which is the
plasma icon

Test Plan: icon lovely now

Reviewers: broulik, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20172