OpenSSL: Update to match the modified DH_get0_key() API

OpenSSL 1.1.0 (master branch) apparently ended up modifying the API
after the beta 2 release that was supposed to complete the work. Mark
the variables const to fix the compilation with the modified OpenSSL
API.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/crypto/crypto_openssl.c

@@ -653,7 +653,8 @@ err:
 	DH *dh;
 	struct wpabuf *pubkey = NULL, *privkey = NULL;
 	size_t publen, privlen;
-	BIGNUM *p = NULL, *g, *priv_key = NULL, *pub_key = NULL;
+	BIGNUM *p = NULL, *g;
+	const BIGNUM *priv_key = NULL, *pub_key = NULL;
 
 	*priv = NULL;
 	wpabuf_free(*publ);