fix nu-system build on arm64 FreeBSD (#13196)

# Description

Fixes #13194

`ki_stat` is supposed to be a `c_char`, but was defined was `i8`.
Unfortunately, `c_char` is `u8` on Aarch64 (on all platforms), so this
doesn't compile. I fixed it to use `c_char` instead.

Double checked whether NetBSD is affected, but the `libc` code defines
it as `i8` for some reason (erroneously, really) but that doesn't matter
too much. Anyway should be ok there.

Confirmed to be working.

crates/nu-system/src/freebsd.rs

@@ -1,6 +1,7 @@
 use itertools::{EitherOrBoth, Itertools};
 use libc::{
- kinfo_proc, sysctl, CTL_HW, CTL_KERN, KERN_PROC, KERN_PROC_ALL, KERN_PROC_ARGS, TDF_IDLETD,
+ c_char, kinfo_proc, sysctl, CTL_HW, CTL_KERN, KERN_PROC, KERN_PROC_ALL, KERN_PROC_ARGS,
+ TDF_IDLETD,
 };
 use std::{
  ffi::CStr,
@@ -16,7 +17,7 @@ pub struct ProcessInfo {
  pub ppid: i32,
  pub name: String,
  pub argv: Vec<u8>,
- pub stat: i8,
+ pub stat: c_char,
  pub percent_cpu: f64,
  pub mem_resident: u64, // in bytes
  pub mem_virtual: u64, // in bytes