Add extern crate libc declaration for Linux

The libc crate needs to be explicitly declared when used on Linux
for getuid() and kill() system calls.

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

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

Author: ammario

src/jail/linux.rs

@@ -1,3 +1,7 @@
+// 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};