Fix: field-class.cpp: calling _option_fc lib function on VariantFieldClass

Fix: field-class.cpp: calling _option_fc lib function on VariantFieldClass

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ie3bbc05a20368b8b1388b484ef02da4fb4b68a2d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7497
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10758
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>