nghttp2 has yet again extended its callback struct and this is an
attempt to make curl compile with nghttp2 from current git
on_unknown_frame_recv, /* nghttp2_on_unknown_frame_recv_callback */
on_begin_headers, /* nghttp2_on_begin_headers_callback */
on_header /* nghttp2_on_header_callback */
+#if NGHTTP2_VERSION_NUM >= 0x000400
+ , NULL /* nghttp2_select_padding_callback */
+#endif
};
static ssize_t data_source_read_callback(nghttp2_session *session,