Add openssl to Build-Depends

Let's see whether this solves:

| MAIN_NAME=kamailio ./sip-router_cert.sh -d /tmp/buildd/kamailio-3.3+ngcp2.8.0+0~1367239454.423+wheezy~1.gbp8b35cf/debian/kamailio-tls-modules/etc/kamailio/
| Could not find openssl command