Fix SPM usage with better publicHeadersPath. (#4554)

Package.swift

@@ -34,4 +34,4 @@ let package = Package(name: "AFNetworking",
                                           targets: ["AFNetworking"])],
                       targets: [.target(name: "AFNetworking",
                                         path: "AFNetworking",
-                                        publicHeadersPath: "AFNetworking")])
+                                        publicHeadersPath: "")])