Fix libsbuf sbuf_printf_drain symbol version

(Introduced incorrectly in r347229 earlier today.)

As pointed out by kevans, 1.6 should be used for FreeBSD 13, like r340383.

Submitted by: kevans
Reported by: kib
Reviewed by: jilles
X-MFC-with: r347229
Differential Revision: https://reviews.freebsd.org/D20187