Install our org.kde.screensaver interface definition

Summary:
KScreenLocker has two interfaces, the FDO interface and a custom
interface with some extensions. The XML definition of one is currently
installed, this adds the second.

Test Plan: Looked at ouptut

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15186