fix unread-fields warnings on latest nightly (#1330)

It's interesting to note that one of these warnings constitute
reading the fields, but these come about by displaying a
debug version of these fields, which now doesn't qualify anymore.

Author: Byron

gix-index/src/extension/fs_monitor.rs

@@ -6,6 +6,7 @@ use crate::{
 };
 
 #[derive(Clone)]
+#[allow(dead_code)]
 pub enum Token {
     V1 { nanos_since_1970: u64 },
     V2 { token: BString },