add .xccheckout to Objective-C gitignore

This is a file that Xcode 5 uses for version control, doesn't need to be checked in. Also see http://stackoverflow.com/a/18448100/1877078

Objective-C.gitignore

@@ -15,6 +15,7 @@ profile
 DerivedData
 .idea/
 *.hmap
+*.xccheckout
 
 #CocoaPods
 Pods