build: disable openssl build warnings on macos

We already disable `-Wmissing-field-initializers` on other Unices but
not on MacOS.

PR-URL: https://github.com/nodejs/node/pull/19046
Fixes: https://github.com/nodejs/node/issues/18983


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>