Fix compile error

[pjc0247]

Unity.iOS is not exist by default.

[KojiNakamaru]

Thank you for your contribution. Your fix is completely logical, but I intentionally don't utilize UNITY_IOS and/or UNITY_ANDROID, as Unity occasionally misses these for PostProcessBuild scripts.

cf. #342 (comment)
cf. https://answers.unity.com/questions/1512004/pbxproject-and-unity-ios-issue-in-postprocessbuild.html

I'll later add comments about this point in README.md.

[pjc0247]

Thanks,

Unity, they always miss something