Summary:
liburing is a Linux-specific dependency, so make sure it's configured in the Linux-only Buck rules.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6378

Test Plan:
```
~/fbcode $ cp internal_repo_rocksdb/repo/TARGETS rocksdb/src
~/fbcode $ buck build mode/mac eden
```

Reviewed By: chadaustin

Differential Revision: D19760039

Pulled By: riversand963

fbshipit-source-id: 2abfce81c8b17965ef76012262cd117708e0294f