Export install location for DBUS interfaces via CMake
Summary:
This is useful for any other package depending on the interface
description at build time, and mimics other packages which export
DBUS interfaces.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D15228