Add X-KDE-OnlyShowOnQtPlatforms to KDEDModule servicetype definition

Summary:
KCoreAddons' DesktopFileParser needs a definition of the types of all
possible entries to do a proper conversion to JSON types.

Test Plan:
kded no longer complains

Expected JSON property "X-KDE-OnlyShowOnQtPlatforms" to be a string list.

when loading kded_touchpad module which has in the metadata

X-KDE-OnlyShowOnQtPlatforms=xcb;

Reviewers: Plasma, mart, apol

Reviewed By: apol

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11123