tls: avoid discards ‘const’ qualifier warning for older openssl

src/tls/openssl/tls.c

@@ -2123,7 +2123,7 @@ struct tls_cert *tls_cert_for_sni(const struct tls *tls, const struct pl *sni)
 	cn = mem_zalloc(sni->l + 1, NULL);
 	LIST_FOREACH(&tls->certs, le) {
 		X509 *x509;
-		const X509_NAME *nm;
+		X509_NAME *nm;
 		bool match = false;
 		int err;