checkstyle9.pl: don't require Signed-off-by for committers

Silence the warning about Signed-off-by: lines if the Author: is using a
FreeBSD.org address. We don't require the lines in direct FreeBSD
commits so the warning is just noise in this case.

Reviwed by: imp, emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/1577