SSL: avoid SSL_CTX_set_tmp_rsa_callback() call with LibreSSL.

LibreSSL removed support for export ciphers and a call to
SSL_CTX_set_tmp_rsa_callback() results in an error left in the error
queue.  This caused alerts "ignoring stale global SSL error (...called
a function you should not call) while SSL handshaking" on a first connection
in each worker process.