[System Tray] Restore old pin button size

Summary: This size was used before the rewrite (D24720).

Test Plan:
Before:
{F7788944}
After:
{F7788943}

Reviewers: #plasma, #vdg, davidedmundson, broulik

Reviewed By: #plasma, davidedmundson, broulik

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25602