Add X-KDE-OnlyShowOnQtPlatforms to KCModule servicetype definition

Summary:
KCoreAddons' DesktopFileParser needs a definition of the types of all
possible entries to do a proper conversion to JSON types.
It is also nicer for plugin developers to have that type documented.

Reviewers: Plasma, mart, apol

Reviewed By: apol

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11125