[touchpad kded] Work around kded fail on X-KDE-OnlyShowOnQtPlatforms lists

Summary:
kded currently fails to handle list values set for X-KDE-OnlyShowOnQtPlatforms
and will read the whole value as one string "xcb;" including the semicolon.

Reviewers: mart, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11126