Fix compile for Xcode 4.6.3, in order to use ios-sim on older OS X systems

I'm not an Objective-C expert, but the following appears to be the correct switch in the ios-sim code, in order to fix compilation with an older version of Xcode.

When compiling ios-sim against newer Xcode versions, we have had errors trying to run the tool on older OS X installs. These changes allow it to be compiled in OS X 10.7 with Xcode 4.6.3, which should help retain compatibility at the desktop level.