Tell AppStream generators not to index Klipper .desktop files

Summary:
Same deal as D23306. We need to tell AppStream generators to exclude these files from
metadata indexing so Discover and other software center apps don't show duplicate
entries. This happens because kpackagetool5 automatically generates appstream metadata
already that marks Klipper as a Plasma addon, so the additional entries are not necessary.

Test Plan: None; can't test AppStream changes. :/

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23320