DolphinQt/Settings: Replace includes with forward declarations

Avoids dragging in netplay-related headers where they aren't explicitly
necessary.

Source/Core/DolphinQt/Settings.h

@@ -11,9 +11,6 @@
 #include <QSettings>
 #include <QVector>
 
-#include "Core/NetPlayClient.h"
-#include "Core/NetPlayServer.h"
-
 namespace Core
 {
 enum class State;
@@ -24,6 +21,12 @@ namespace DiscIO
 enum class Language;
 }
 
+namespace NetPlay
+{
+class NetPlayClient;
+class NetPlayServer;
+}
+
 class GameListModel;
 class InputConfig;
 class QFont;
@@ -39,6 +42,8 @@ public:
   Settings(Settings&&) = delete;
   Settings& operator=(Settings&&) = delete;
 
+  ~Settings();
+
   static Settings& Instance();
   static QSettings& GetQSettings();