Fixed warning in UISSUserInterfaceIdiomPreprocessor.m

Xcode 5 reports the following warning:
```
UISSUserInterfaceIdiomPreprocessor.m:25:19: Comparison of constant 'NSNotFound' (2147483647) with expression of type 'UIUserInterfaceIdiom' (aka 'enum UIUserInterfaceIdiom') is always false
```

Fixed by no longer casting `NSInteger` (possibly holding a non-`UIUserInterfaceIdiom` value) to `UIUserInterfaceIdiom`.