[infra] Add ninja to the SDK DEPS
[infra] Add ninja to the SDK DEPS

This will allow us to gradually migrate from depot_tools ninja to our
own. See also https://crbug.com/1340825.

Bug: https://github.com/dart-lang/sdk/issues/50032
Change-Id: I5118c98abcb4399ec5d952c5a3e240f8e3803475
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/260762
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>
Reviewed-by: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk