Fix iOS build by excluding external CoreML SDK dependencies (#3043)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3043

CoreML delegate SDK integration broke the app build. Getting the SDK integration to work properly internally will require buckifying the third-party targets on which the CoreML delegate SDK itself depends (not to be confused with the third-party dependencies from ET itself). Running the `install_requirements.sh` script (CoreML's, not the generic ET one) clones a bunch of Git repos, XCode-specific tooling, and generates Protobuf headers on which their SDK integration relies.

To avoid this, we simply add the `BUILD_SDK` flag and set it to false and disable building the SDK and exclude references to generated headers.

Reviewed By: kirklandsign

Differential Revision: D55456558

fbshipit-source-id: 6ab931b39298ee0a4a4b238699c64c84952e180e