Fix IOSResolver platforms. Use only on mac

[RevenantX]

Set correct platforms for IOSResolver (only mac editor)
Fixes:

• [Bug] IOSResolver.dll tries to load on Windows/Linux - causes error #622
• [BUG] Unable to resolve reference 'UnityEditor.iOS.Extensions.Xcode' on Windows #412

Since you cannot fully build IOS builds on windows this change is totally safe

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[StephenHodgson]

@RevenantX actually you can build mac/iOS on windows/linux, just not IL2CPP for mac standalone, but you can def export iOS projects. The real fix can be found in #714

[RevenantX]

@StephenHodgson you can build Mac on Windows (Mono) but not iOS. Because iOS is 64bit only - you need IL2CPP and XCode to actually build iOS application.

As you see Build button is grayed.

[StephenHodgson]

As you see Build button is grayed.

Build and Run is greyed out yes, but you can still build and export the xcode project itself.
Regardless your comment about being able to build on windows is incorrect. I do this all the time.

Believe, I know quite a lot about the build system. You should check out my automated build tasks for unity 😉

Even so, my PR will properly update the json config for exporting the package, which is really what should be done in this case to conform with how the project is setup and deployed.