RESPONDER: Don't expose client_idle_handler()
    
    It's been only used by reset_client_idle_handler(). So, no need to
    expose this function.
    
    Related:
    https://fedorahosted.org/sssd/ticket/3245
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>