deps,src: align ssize_t ABI between Node & nghttp2

Previously, we performed casts that are considered undefined behavior.
Instead, just define `ssize_t` for nghttp2 the same way we define it
for the rest of Node.

Also, remove a TODO comment that would probably also be *technically*
correct but shouldn’t matter as long as nobody is complaining.

Backport-PR-URL: https://github.com/nodejs/node/pull/20456
PR-URL: https://github.com/nodejs/node/pull/18565


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>