VideoCommon/NetPlayGolfUI: Default the destructor in the cpp file

Ensures the destruction logic is kept local to the translation unit. It
also doesn't really do much to have it specified in the header.

Source/Core/VideoCommon/NetPlayGolfUI.cpp

@@ -20,6 +20,8 @@ NetPlayGolfUI::NetPlayGolfUI(std::shared_ptr<NetPlay::NetPlayClient> netplay_cli
 {
 }
 
+NetPlayGolfUI::~NetPlayGolfUI() = default;
+
 void NetPlayGolfUI::Display()
 {
   auto client = m_netplay_client.lock();