Use BALOO_DBUS_INTERFACES_DIR

Summary:
...to find baloo's dbus interfaces. It's defined in
KF5BalooConfig.cmake and should always have the correct location of
the interface files, avoiding the trickery with CMAKE_INSTALL_PREFIX
which might easily go wrong with a custom prefix.

Test Plan: Builds, indexing via baloo works

Reviewers: mgallien

Differential Revision: https://phabricator.kde.org/D7592