FilesystemWidget: Replace DiscIO includes with forward declarations

Avoids the need to use an ugly ifdef around a header, and also resolves
an indirect inclusion in PropertiesDialog.

Source/Core/DolphinQt2/Config/FilesystemWidget.h

@@ -7,12 +7,6 @@
 #include <QDialog>
 #include <memory>
 
-// Qt versions prior to 5.9 don't support C++17 nested namespaces in moc so we have to if-guard
-// this header
-#ifndef Q_MOC_RUN
-#include "DiscIO/Volume.h"
-#endif
-
 #include "UICommon/GameFile.h"
 
 class QStandardItem;
@@ -22,14 +16,17 @@ class QTreeView;
 namespace DiscIO
 {
 class FileInfo;
+class Volume;
+
 struct Partition;
-};  // namespace DiscIO
+}  // namespace DiscIO
 
 class FilesystemWidget final : public QWidget
 {
   Q_OBJECT
 public:
   explicit FilesystemWidget(const UICommon::GameFile& game);
+  ~FilesystemWidget() override;
 
 private:
   void CreateWidgets();