Fix build with opaque structs in LibreSSL 3.5

Index: src/test/test_crypto.c
--- src/test/test_crypto.c.orig
+++ src/test/test_crypto.c
@@ -187,7 +187,7 @@ test_crypto_dh(void *arg)
     dh4 = crypto_dh_new_openssl_tls();
     tt_assert(DH_generate_key(dh4));
     const BIGNUM *pk=NULL;
-#ifdef OPENSSL_1_1_API
+#if defined(OPENSSL_1_1_API) || defined(LIBRESSL_VERSION_NUMBER)
     const BIGNUM *sk=NULL;
     DH_get0_key(dh4, &pk, &sk);
 #else
