VideoCommon/NetPlayChatUI: Default destructor in the cpp file

Ensures that the destruction logic is kept local to the translation unit
(making it nicer when it comes to forward declaring non-trivial types).
It also doesn't really do much to define it in the header.

Source/Core/VideoCommon/NetPlayChatUI.h

@@ -15,7 +15,7 @@ class NetPlayChatUI
 {
 public:
   explicit NetPlayChatUI(std::function<void(const std::string&)> callback);
-  ~NetPlayChatUI() = default;
+  ~NetPlayChatUI();
 
   using Color = std::array<float, 3>;