Fix vk_mvk_moltenvk.h on macOS 10.12 and earlier

[evanmiller]

IOSurfaceRef.h does not exist on macOS 10.12 and earlier. However the IOSurfaceRef type will be indirectly included by the Metal include, so we just need to ensure IOSurfaceRef is defined when __OBJC__ is not. This can be done with a typedef.

See: https://trac.macports.org/ticket/67263

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[evanmiller]

Actually, I'm told this patch doesn't work correctly on 10.9 so it may need some tweaking.

[Benau]

It should be done in https://github.com/KhronosGroup/MoltenVK anyway