Fix port/pg_iovec.h building extensions on x86_64-darwin

When building pg_cron [1] or pg_hll [2] for PG 17 on x86_64-darwin, we
encounter the following build failure in nixpkgs:

/nix/store/2clw9wg933c40f871d8iqbl909lg2yis-postgresql-17.0-dev/include/server/port/pg_iovec.h:71:12:
error: comparison of integers of different signs: 'ssize_t' (aka 'long')
and 'const size_t' (aka 'const unsigned long') [-Werror,-Wsign-compare]
if (part < iov[i].iov_len)
~~~~ ^ ~~~~~~~~~~~~~~
/nix/store/2clw9wg933c40f871d8iqbl909lg2yis-postgresql-17.0-dev/include/server/port/pg_iovec.h:110:12:
error: comparison of integers of different signs: 'ssize_t' (aka 'long')
and 'const size_t' (aka 'const unsigned long') [-Werror,-Wsign-compare]
if (part < iov[i].iov_len)
~~~~ ^ ~~~~~~~~~~~~~~
2 errors generated.

The attached patch fixes those.

Hopefully this can make it into the minor release next week.

Best,

Wolfgang

[1]: https://hydra.nixos.org/build/276421287/nixlog/1
[2]: https://hydra.nixos.org/build/276419879/nixlog/1