VideoCommon/NetPlayChatUI: Take std::string by value in AppendChat()

Given we're simply storing the std::string into a deque. We can emplace
it and move it. Completely avoiding copies with the current usage of the
function.

Source/Core/VideoCommon/NetPlayChatUI.h

@@ -20,7 +20,7 @@ public:
   using Color = std::array<float, 3>;
 
   void Display();
-  void AppendChat(const std::string& message, Color color);
+  void AppendChat(std::string message, Color color);
   void SendMessage();
   void Activate();