kolab-util-backend: build fixes, cleanup
kolab-util-backend: build fixes, cleanup

* commented out kolab_util_backend_get_relative_path_from_uri()
  (to be re-implemented if needed)
* code formatting
* include camel-system-headers.h from libekolabutil
  instead of single Camel headers (which is now disallowed)

GNOME/evolution-kolab,GNOME/evolution-kolab