Fix build with opaque structs in LibreSSL 3.5

Index: src/lib/tls/x509_openssl.c
--- src/lib/tls/x509_openssl.c.orig
+++ src/lib/tls/x509_openssl.c
@@ -329,7 +329,7 @@ tor_tls_cert_is_valid(int severity,
   cert_key = X509_get_pubkey(cert->cert);
   if (check_rsa_1024 && cert_key) {
     RSA *rsa = EVP_PKEY_get1_RSA(cert_key);
-#ifdef OPENSSL_1_1_API
+#if defined(OPENSSL_1_1_API) || defined(LIBRESSL_VERSION_NUMBER)
     if (rsa && RSA_bits(rsa) == 1024) {
 #else
     if (rsa && BN_num_bits(rsa->n) == 1024) {
