Remove extern crate libc - not needed in Rust 2018+

The extern crate syntax is deprecated in Rust 2018+ edition.
The libc crate is already listed in Cargo.toml and can be used
directly with :: paths.

🤖 Generated with Claude Code (https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ammario

src/jail/linux.rs

@@ -1,7 +1,3 @@
-// libc is needed for getuid() and kill() system calls on Linux
-#[cfg(target_os = "linux")]
-extern crate libc;
-
 use super::{Jail, JailConfig};
 use anyhow::{Context, Result};
 use std::process::{Command, ExitStatus};