Stream: avoid SSL_CTX_set_tmp_rsa_callback() call with LibreSSL.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

src/stream/ngx_stream_ssl_module.c

diff --git a/src/stream/ngx_stream_ssl_module.c b/src/stream/ngx_stream_ssl_module.c
index 97a0fa9..7abd9e1 100644
--- a/src/stream/ngx_stream_ssl_module.c
+++ b/src/stream/ngx_stream_ssl_module.c
@@ -276,7 +276,9 @@
         SSL_CTX_set_options(conf->ssl.ctx, SSL_OP_CIPHER_SERVER_PREFERENCE);
     }
 
+#ifndef LIBRESSL_VERSION_NUMBER
     SSL_CTX_set_tmp_rsa_callback(conf->ssl.ctx, ngx_ssl_rsa512_key_callback);
+#endif
 
     if (ngx_ssl_dhparam(cf, &conf->ssl, &conf->dhparam) != NGX_OK) {
         return NGX_CONF_ERROR;