[Desktop theme KCM] Use new icon

Summary: Icon was just introduced with D18433. The new icon's name is such that the fallback icon will be `preferences-desktop-plasma`, which is already present in 3rd-party icon themes, so nothing will get broken.

Test Plan: {F6561083}

Reviewers: #vdg, #plasma, GB_2

Reviewed By: #vdg, GB_2

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18441