I already answered this in your other post. Is there an Android sharedpreferences equivalent in iOS to save credentials (Username and Password)?
I'll repost it here:
This happens because you are probably using "ARC". Are you? If so, you can set a flag to ignore ARC for that specific file. Select your project, Select your target, select Build Phases and expand the "Compile Sources" drop down. Look for KeychainItemWrapper (or whatever you call your keychain file) and set this flag. -fno-objc-arc