Fix "metal_delegate.h" missing file error on iOS for TFlite 2.3.0+

[swarmidentity]

Add explicit reference to iOS metal headers

[swarmidentity]

See:

https://github.com/shaqian/flutter_tflite/issues/139 https://github.com/shaqian/flutter_tflite/issues/184 https://www.tensorflow.org/lite/performance/gpu#ios_with_xcode https://github.com/am15h/tflite_flutter_plugin/issues/100

Seems like on TFLite 2.3.0+ metal.h is in a different package.