Add iOS-CODE_SIGN_IDENTITY for OSX-build-scheme as well.

This pull request will fix CodeSign error on xcodebuild when linked by depending framework (e.g. SwiftTask) and building as `sdk=iphoneos` but `SwiftState-OSX` is sometimes associated by xcworkspace.

See ReactKit/SwiftTask/pull/13 for more info.

This fix is basically the same as what Carthage/LlamaKit is doing in https://github.com/Carthage/LlamaKit/commit/5b9d9a4db21dc8e9ac1f5f59f3b8d93f92a90eee.