Fix a future-compat warning
Fix a future-compat warning

```
warning[E0506]: cannot assign to `self.input.cached_token` because it is borrowed
   --> src/parser.rs:591:17
    |
566 |     pub fn next_including_whitespace_and_comments(&mut self) -> Result<&Token<'i>, BasicParseError<'i>> {
    |                                                   - let's call the lifetime of this reference `'1`
...
579 |             Some(ref cached_token)
    |                  ---------------- borrow of `self.input.cached_token` occurs here
...
591 |                 self.input.cached_token = Some(CachedToken {
    |                 ^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `self.input.cached_token` occurs here
...
603 |         Ok(token)
    |         --------- returning this value requires that `self.input.cached_token.0` is borrowed for `'1`
    |
    = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
    = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
```

servo/rust-cssparser,servo/rust-cssparser