Fix coreml runner build (#2536)

Summary:
The runner fails to build
```
/Users/runner/work/executorch/executorch/pytorch/executorch/backends/apple/coreml/runtime/delegate/ETCoreMLModelManager.mm:387:43: error: no member named 'ModelPackage' in '(anonymous namespace)::ModelAssetType'
    if (modelAssetType == ModelAssetType::ModelPackage) {
```

The PR fixes the issue. Tested all CoreML builds.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2536

Reviewed By: kirklandsign

Differential Revision: D55178665

Pulled By: shoumikhin

fbshipit-source-id: 79d94e7bcd7e27c4a152bcdb1c9054e2bbbc1718