fix: the build with feature 'socket' on macOS

[SteveLauC]

What does this PR do

Before this PR, I could not build Nix with feature socket enabled on macOS:

$ cargo b --features socket -q
error: struct `GetCString` is never constructed
    --> src/sys/socket/sockopt.rs:1700:8
     |
1700 | struct GetCString<T: AsMut<[u8]>> {
     |        ^^^^^^^^^^
     |
note: the lint level is defined here
    --> src/lib.rs:50:9
     |
50   | #![deny(unused)]
     |         ^^^^^^
     = note: `#[deny(dead_code)]` implied by `#[deny(unused)]`

error: could not compile `nix` (lib) due to 1 previous error

I am using Rust 1.81, this was not caught in our CI because it won't happen with Rust 1.69:

$ cargo +1.69 b --features socket -q

$ echo $?
0

This PR fixes the issue.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API