Fix compile error & redundant class qualifier.

Author: chris-hatton

Diff for: src/MqttClient.cpp

@@ -98,7 +98,7 @@ MqttClient::~MqttClient()
 }
 
 #ifdef MQTT_CLIENT_STD_FUNCTION_CALLBACK
-void MqttClient::onMessage(MqttMessageCallback callback)
+void MqttClient::onMessage(MessageCallback callback)
 #else
 void MqttClient::onMessage(void(*callback)(int))
 #endif

Diff for: src/MqttClient.h

@@ -141,7 +141,7 @@ class MqttClient : public Client {
   Client* _client;
 
 #ifdef MQTT_CLIENT_STD_FUNCTION_CALLBACK
-  MqttClient::MessageCallback _onMessage;
+  MessageCallback _onMessage;
 #else
   void (*_onMessage)(int);
 #endif