NSColor+MGLAdditions.hpp is public

[1ec5]

Building the macosapp target triggers the following compiler warning:

Lexical or Preprocessor Issue Group
/path/to/mapbox-gl-native/platform/macos/<module-includes>:1:1: Umbrella header for module 'Mapbox' does not include header '/path/to/mapbox-gl-native/platform/macos/src/NSColor+MGLAdditions.hpp'

This Objective-C++ header should not be public.

/cc @frederoni