Fix metal build by adding a separate CPP target

[bjnortier]

Not really sure about this but opening a PR for discussion.

Building the SPM package with metal generates a bunch of NSString etc. errors.

If you move whisper.cpp into a separate target it solves the problem, so it looks like an issue with combining CPP and C/ObjC files (but it works without metal?).

There could be another way to solve it in a single target... looking into it.

[bjnortier]

Some other observations:

1. The package works fine when imported into a project in Xcode, but using swift build fails.
2. Apple recommends using two targets for bundling Metal sources with C sources etc. (but it's a but different): https://developer.apple.com/documentation/technotes/tn3133-packaging-a-renderer
3. It's adding the ggml-metal.metal file the causes the errors.

[bjnortier]

Closed because whisper.cpp didn't have -DGGML_USE_METAL and the test crashes when I add it