BUILD/MINOR: ssl: fix compiler warning about useless statement

There is a compiler warning after commit a9363eb6 ("BUG/MEDIUM: ssl:
'tune.ssl.default-dh-param' value ignored with openssl > 1.1.1"):

src/ssl_sock.c: In function 'ssl_sock_prepare_ctx':
src/ssl_sock.c:4481:4: error: statement with no effect [-Werror=unused-value]

Fix it by adding a (void)

src/ssl_sock.c

@@ -5041,7 +5041,7 @@ int ssl_sock_prepare_ctx(struct bind_conf *bind_conf, struct ssl_bind_conf *ssl_
 			 NULL);
 
 		if (ecdhe == NULL) {
-			SSL_CTX_set_ecdh_auto(ctx, 1);
+			(void)SSL_CTX_set_ecdh_auto(ctx, 1);
 			return cfgerr;
 		}
 #else