http2: fix compiling with `--debug-http2`

PR-URL: https://github.com/nodejs/node/pull/17863
Fixes: https://github.com/nodejs/node/issues/17840


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>