catar/encoder.rs: allow to pass list of devices

We can use this to implement the --one-file-system like options. This
is a bit more convenient, because this way we can select to include
multiple devices (which is required by container backups).

src/bin/proxmox-backup-client.rs

@@ -40,7 +40,7 @@ fn backup_dir(
 
     let path = std::path::PathBuf::from(path);
 
-    CaTarEncoder::encode(path, dir, &mut index)?;
+    CaTarEncoder::encode(path, dir, None, &mut index)?;
 
     index.close()?; // commit changes