res_rtp_asterisk: Fix DTLS when used with OpenSSL 1.0.1k

A recent security fix for OpenSSL broke DTLS negotiation for many
applications. This was caused by read ahead not being enabled when it
should be. While a commit has gone into OpenSSL to force read ahead
on for DTLS it may take some time for a release to be made and the
change to be present in distributions (if at all). As enabling read
ahead is a simple one line change this commit does that and fixes
the issue.

ASTERISK-24711 #close
Reported by: Jared Biel


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@431384 65c4cc65-6c06-0410-ace0-fbb531ad65f3