Fix: lib/graph/iterator: consider clock classes from message iterator inactivity...

Fix: lib/graph/iterator: consider clock classes from message iterator inactivity messages for clock correlation validation

Make the clock correlation validation code consider clock classes from
message iterator inactivity messages.  This is equivalent to what the
muxer component class does (see `MsgIter::_validateMsgClkCls()`).

Change-Id: I172d0015eb45508da7c412172b24aa4b0236ae9f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12120
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>