FreeBSD: fix world build after 143476ce8

Do not redefine the fallthrough macro when building with libcpp.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes #12880