[llvm] [unused-includes][ADT] Don't include DJB.h as it's not used. NFC. (PR #88211)

[Volodymyr Sapsai via llvm-commits]

vsapsai wrote:

Buildkite reports
```
Total Discovered Tests: 36933
  Skipped          :    30 (0.08%)
  Unsupported      :   243 (0.66%)
  Passed           : 36624 (99.16%)
  Expectedly Failed:    36 (0.10%)
```
but still decided the build has failed.

Don't know how to re-trigger the tests.

https://github.com/llvm/llvm-project/pull/88211