Silence warnings when building via CocoaPods 0.21 or later

CocoaPods 0.21 changed how pods are built; instead of being mashed into one big target, each is build in its own target, which are then linked together. As a result of this, AFAmazonS3Client doesn't see the prefix header bits added by AFNetworking, and the warnings about missing frameworks appear. This change is a bit of a hack to silence said warnings.