Document two librsvg2 vulnerabilities
PR: 205502
Security: CVE-2015-7557
Security: CVE-2015-7558
Security: https://vuxml.FreeBSD.org/freebsd/da634091-a84a-11e5-8f5c-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/d6c51737-a84b-11e5-8f5c-002590263bf5.html