security/clamassassin: fix non-compliant email header
The X-Virus-Checker-Version: header added by clamassassin contains a newline if
clamd is down. This creates a non-compliant email header (because it doesn't
end with a colon).
PR: 261131
Reported by: martin@lispworks.com
Approved by: chalpin@cs.wisc.edu (maintainer, timeout > 1 week)