src: do not use posix feature macro in node.h

This macro is only defined when building Node.js, so addons cannot
use it as a way of detecting feature availability.

PR-URL: https://github.com/nodejs/node/pull/27775
Refs: https://github.com/nodejs/node/pull/27246


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>