fix(netbsd): remove IFF_NOTRAILERS

[fujiapple852]

libc version 0.2.178 includes this change:

NetBSD: Remove IFF_NOTRAILERS (rust-lang/libc#4782)

This causes nix to fail to compile against NetBSD targets:

$ cargo check --target x86_64-unknown-netbsd
    Checking nix v0.30.1
error[E0425]: cannot find value `IFF_NOTRAILERS` in crate `libc`

Real world failure observed in Gitlab CI using cargo-cross: https://github.com/fujiapple852/trippy/actions/runs/20000766159/job/57355635047#step:7:402

This MR is similar to #893 which removed IFF_NOTRAILERS from OpenBSD.

aside: I didn't add this to the CHANGELOG.md as there was no unreleased section.