BUILD: ssl: avoid a warning about conn not used with OpenSSL < 1.0.1

BUILD: ssl: avoid a warning about conn not used with OpenSSL < 1.0.1

Building with a version of openssl without heartbeat gives this since
latest 29f037d ("MEDIUM: ssl: explicitly log failed handshakes after a
heartbeat") :

src/ssl_sock.c: In function 'ssl_sock_msgcbk':
src/ssl_sock.c:188: warning: unused variable 'conn'

Simply declare conn inside the ifdef. No backport is needed.