cp: disable "extra_unused_lifetimes" lint

[cakebaker]

With the new Rust version there is now a clippy warning:

warning: this lifetime isn't used in the impl
   --> src/uu/cp/src/cp.rs:61:1
    |
61  | / quick_error! {
62  | |     #[derive(Debug)]
63  | |     pub enum Error {
64  | |         /// Simple io::Error wrapper
...   |
105 | |     }
106 | | }
    | |_^

This PR simply disables the corresponding lint as I don't know how to fix it otherwise.

[tertsdiepraam]

I opened a PR on quick_error to fix this issue. Hopefully they merge it and then we can remove this again. :)