Add automatically applicable suggestion to unused_label

[killercup]

Found by running rustfix on https://github.com/space-wizards/bsdiff-rs.

I suggest suggesting (heh) to replace 'loop7: with an empty string. (Might need special casing in diagnostics rendering?)

warning: unused label `'loop7`
   --> src/diff.rs:468:13
    |
468 | /             'loop7: loop {
469 | |                 if !(scsc < scan + len) {
470 | |                     break;
471 | |                 }
...   |
477 | |                 scsc += 1isize;
478 | |             }
    | |_____________^
    |
    = note: #[warn(unused_label)] on by default
    = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#unused_label