audit(4): fix the definition of ARG_TERMID_ADDR

Due to a copy/paste error in r168688, ARG_TERMID_ADDR has the same
definition as ARG_SADDRUNIX. Fix it.

The header change, while publicly visible, is guarded by #ifdef KERNEL, and
I can't find any kmod ports that use it. So I'm not bumping
__FreeBSD_version.

PR: 228820
Submitted by: aniketp
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15702