fix Ring dbus-xml interfaces path auto-discovery

If LRC is build with a CMAKE_INSTALL_PREFIX equivalent
to where the daemon is installed, the build fails,
without indicating where the daemon has been installed.

This is due to incorrect given path (was api, should be interface )

Change-Id: Iab2e2118ae4a82edc0ad386bbe72048bd9a94538
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>