src: inline SetSNICallback

Refs: https://github.com/nodejs/node/pull/30548#discussion_r348168855

PR-URL: https://github.com/nodejs/node/pull/30548


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>