lib: plugin-dev.h: rename `MESSAGE_ITERATOR` -> `MESSAGE_ITERATOR_CLASS`

This is more in line with the message iterator class concept and
indicates that a given method belongs to the (implicit) component
class's message iterator class.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Icbcefec886fcbb2b1928d4b1009f3aca88c032a0
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2751
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>