Add license symlink

[returntrip]

To make it easier for Linux distributions to ship the licenses text within the rustc_tools_util crate directory.

changelog: none

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @phansch (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[phansch]

Seems unproblematic to me to add these symlinks, but to be sure I'll defer to @matthiaskrgr who's the owner of the crate

r? @matthiaskrgr

[matthiaskrgr]

Looks fine to me, thanks.
@bors r+

[bors]

📌 Commit f2154e9 has been approved by matthiaskrgr

[bors]

⌛ Testing commit f2154e9 with merge f28c6de...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: matthiaskrgr
Pushing f28c6de to master...