The attached patch fixes build error with latest nghttp2.

nghttp2 changed its interfaces:
1. replace nghttp2_error_code parameter in callback function to uint32_t
2. callbacks struct is now hidden from application API

The reason of 1 is that h2-14 spec now allows extension to define its own
error code. To propagate such error code, we need to use uint32_t instead
of enum types.

The reason of 2 is that it is most likely we are going to add new callbacks
(recently we added 2 callbacks). To avoid so name bump on each addition of
callback, we decided to hide the details of the callback struct and provide
the setter like functions to set callbacks.

Best regards,
Tatsuhiro Tsujikawa

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