[Issue #1833] Podspec fix.

"pod spec lint" will fail with the following error unless we include the dependency on NSURLSession in the UIKit sub spec:

- ERROR | [iOS] [AFNetworking/UIKit] [xcodebuild]  AFNetworking/UIKit+AFNetworking/AFNetworkActivityIndicatorManager.m:30:9: fatal error: 'AFURLSessionManager.h' file not found