build: display HTTP2 configure --help options

Currently the options available for HTTP2 are not displayed when running
configure --help.

This commit enables the following HTTP2 section to be included in the
help output:

HTTP2:
  Flags that allows you to control HTTP2 features in Node.js

  --debug-http2       build with http2 debug statements on (default is
                      false)
  --debug-nghttp2     build nghttp2 with DEBUGBUILD (default is false)

PR-URL: https://github.com/nodejs/node/pull/15198


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>