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.cpp

@@ -18,6 +18,8 @@ NetPlayChatUI::NetPlayChatUI(std::function<void(const std::string&)> callback)
 {
 }
 
+NetPlayChatUI::~NetPlayChatUI() = default;
+
 void NetPlayChatUI::Display()
 {
   const float scale = ImGui::GetIO().DisplayFramebufferScale.x;