plugins/common/muxing: compare "origin is Unix epoch" property of clock classes

If two clock classes only differ by their "origin is Unix epoch"
property, they will compare equal as far as `compare_clock_classes` is
concerned.  Add a comparison for that.

Change-Id: Iecac8fd4aa3064b221291264f4fc6758e32a0943
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12067
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>