ctf: bt_ctf_notif_iter_get_next_notification(): require CC prio. map

Since bt_notification_event_create() freezes its clock class priority
map, pass which clock class priority map to use each time you call
bt_ctf_notif_iter_get_next_notification().

In ctf.fs, it's always the same because all the clock classes are known
when the trace object is created.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>