bt2: rename native_bt_notifier.i to native_bt_message_iterator.i

bt2: rename native_bt_notifier.i to native_bt_message_iterator.i

We don't use the term notifier anymore, message iterator is more
contemporary.

Change-Id: Iadda98ac83a2dfddcb207d3beb99ddaa65181ad3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1536
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>