Added vxvml entry for security/dropbear

PR: 220158
Submitted by: Piotr Kubaj <pkubaj@anongoth.pl> (maintainer)
Reviewed by: lifanov (mentor)
Approved by: lifanov (mentor)
MFH: 2017Q3
Security: http://www.vuxml.org/freebsd/60931f98-55a7-11e7-8514-589cfc0654e1.html
Differential Revision: https://reviews.freebsd.org/D11400