meta: add mailmap entry for ratracegrad

There are two AUTHORS entries for ratracegrad. Use a mailmap entry to
consolidate them into a single entry.

PR-URL: https://github.com/nodejs/node/pull/40291


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>