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.cpp

@@ -19,8 +19,9 @@
 #include <future>
 
 #include "DiscIO/DiscExtractor.h"
-#include "DiscIO/Enums.h"
 #include "DiscIO/Filesystem.h"
+#include "DiscIO/Volume.h"
+
 #include "DolphinQt2/QtUtils/ActionHelper.h"
 #include "DolphinQt2/Resources.h"
 
@@ -45,6 +46,8 @@ FilesystemWidget::FilesystemWidget(const UICommon::GameFile& game)
   PopulateView();
 }
 
+FilesystemWidget::~FilesystemWidget() = default;
+
 void FilesystemWidget::CreateWidgets()
 {
   auto* layout = new QVBoxLayout;