Install kservice_version.h in KService include prefix

This is needed for any other repo that needs to include that header; this worked auto-magically for KF5, but doesn't work for KF6.

This is considered compatible as both prefixes are included in the build metadata (thanks to Volker for this wording :)).

For more details see: https://mail.kde.org/pipermail/kde-frameworks-devel/2022-January/120608.html