syslogd: fix WITHOUT_INET builds

Since 2d82b47 syslogd can't be built with WITHOUT_INET or
WITHOUT_INET6 build variables set, because iovlist_truncate is not
defined but used.

This change wraps the problematic iovlist_truncate call within ifdef
directive. It's compiled out in this situation...

Pull Request: https://github.com/freebsd/freebsd-src/pull/475
Reviewed by: imp@ (commit message slightly tweaked)

(cherry picked from commit f06fa6f88733d709a5f9dd3a27b1a08571e49e6b)

PR: 260098