Make it possible to compile on FreeBSD by conditionally using

std::os::<linux,freebsd>::fs::MetadataExt. Not intending to use hpk on
FreeBSD but this enables development from either platform

src/tar/header.rs

@@ -8,10 +8,16 @@ use std::{
     fs::{self, Metadata},
     io,
     ops::Deref,
-    os::{linux::fs::MetadataExt, unix::fs::FileTypeExt},
+    os::unix::fs::FileTypeExt,
     path::PathBuf,
 };
 
+#[cfg(target_os = "linux")]
+use std::os::linux::fs::MetadataExt;
+
+#[cfg(target_os = "freebsd")]
+use std::os::freebsd::fs::MetadataExt;
+
 #[repr(u8)]
 pub enum FileType {
     Normal = 0x30,