The extern declaration is only for Linux, move this line
into the right #ifdef section.
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Co-authored-by: Martin Matuska <mm@FreeBSD.org>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes#13934Closes#13936