job.c: add missing notifier initialization

job.c: add missing notifier initialization

It seems that on_idle list is not properly initialized like
the other notifiers.

Fixes: 34dc97b9a0e ("blockjob: Wake up BDS when job becomes idle")
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>