make EVP_PKEY opaque

Reviewed-by: Richard Levitte <levitte@openssl.org>
diff --git a/ssl/ssl_cert.c b/ssl/ssl_cert.c
index 75ccc72..2aaf99c 100644
--- a/ssl/ssl_cert.c
+++ b/ssl/ssl_cert.c
@@ -214,7 +214,7 @@
 
         if (cpk->privatekey != NULL) {
             rpk->privatekey = cpk->privatekey;
-            CRYPTO_add(&cpk->privatekey->references, 1, CRYPTO_LOCK_EVP_PKEY);
+            EVP_PKEY_up_ref(cpk->privatekey);
         }
 
         if (cpk->chain) {