Fix rounded borders

Summary: This makes borders of elements using plasmoidHeading rounded again

Test Plan:
After:

Clearing cache might be needed here.

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29479

(cherry picked from commit 1bc004e5c65f3e9b84bce4caab9071f77cd34df1)