Fix mismatched_lifetime_syntaxes warning

[9999years]

warning: hiding a lifetime that's elided elsewhere is confusing
   --> src/lru_list.rs:109:17
    |
109 |     pub fn iter(&self) -> LRUListIterator<T> {
    |                 ^^^^^     ------------------ the same lifetime is hidden here
    |                 |
    |                 the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
109 |     pub fn iter(&self) -> LRUListIterator<'_, T> {
    |                                           +++