Fixes 2.0 podspec for mac

When I run `pod spec lint` on the spec, i got compile error for osx. 

```
 -> AFNetworking (2.0.0-RC2)
    - ERROR | [OSX] [xcodebuild]  AFNetworking/UIKit+AFNetworking/UIActivityIndicatorView+AFNetworking.h:23:9: fatal error: 'UIKit/UIKit.h' file not found
    - ERROR | [OSX] [xcodebuild]  AFNetworking/UIKit+AFNetworking/UIProgressView+AFNetworking.h:23:9: fatal error: 'UIKit/UIKit.h' file not found
    - ERROR | [OSX] [xcodebuild]  AFNetworking/UIKit+AFNetworking/UIWebView+AFNetworking.h:23:9: fatal error: 'UIKit/UIKit.h' file not found
```

I believe what the spec want is only include UIKit classes for iOS. The PR should do that.