update a method signature for newer RN versions

Author: rformato

ios/MqttClient.swift

@@ -322,8 +322,7 @@ class MqttClient : RCTEventEmitter {
     }
 
     // https://stackoverflow.com/a/38161889
-    //override func invalidate() -> Void {
-    func invalidate() -> Void {
+    override func invalidate() -> Void {
         os_log("MqttClient: invalidating")
         self.disconnect()
     }