greybus: svc: unexport eject helper

Do no export the interface-eject helper, which is only supposed to be
used by core.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Jeffrey Carlyle <jcarlyle@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>