Add missing USES+=iconv because most of Mailutils programs use functions
it provides, but dependency was not recorded in the package. This bug
went unnoticed for a long time because `libiconv.so.2' library typically
exists on any FreeBSD installation, and was revealed by recent exp-run.

Since port revision was bumped just now in the previous commit, abuse it
and do not bump again.

PR: 229613