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: nodejs#15198
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

configure

@@ -420,6 +420,8 @@ http2_optgroup.add_option('--debug-nghttp2',
     dest='debug_nghttp2',
     help='build nghttp2 with DEBUGBUILD (default is false)')
 
+parser.add_option_group(http2_optgroup)
+
 parser.add_option('--with-perfctr',
     action='store_true',
     dest='with_perfctr',