relicense `shootout-threadring.rs` to the shootout license

[TeXitoi]

See #14248

I propose to change the license of shootout-threadring.rs

Non trivial contributors:

• @brson: OK
• @tedhorst: OK
• @TeXitoi: OK

Contributors appearing in git blame that are trivial (trivial update due to language changes):

• @kmcallister: OK
• @alexcrichton: OK
• @kballard: OK
• @pcwalton: OK

Contributors with removed and trivial contributions are not considered.

For the persons that are not in "OK" state, please respond to this issue saying:

I agree to relicense any previous contributions to http://github.com/rust-lang/rust according to the term of the Computer Language Benchmarks Game license (http://benchmarksgame.alioth.debian.org/license.php)

Then, I'll propose the corresponding PR.

Thanks.

[kmcallister]

I agree to relicense any previous contributions to http://github.com/rust-lang/rust according to the term of the Computer Language Benchmarks Game license (http://benchmarksgame.alioth.debian.org/license.php)

[tedhorst]

I agree to relicense any previous contributions to http://github.com/mozilla/rust according to the term of the Computer Language Benchmarks Game license (http://benchmarksgame.alioth.debian.org/license.php)