Port dump XFB option to Qt UI.

Looks like the option was added to the Wx UI at commit 198d3b69, which
was a few months after the advancedWidget was originally ported from
Wx to Qt, but before anyone was actually using Qt.

Source/Core/DolphinQt/Config/Graphics/AdvancedWidget.h

@@ -40,6 +40,7 @@ private:
   // Utility
   GraphicsBool* m_prefetch_custom_textures;
   GraphicsBool* m_dump_efb_target;
+  GraphicsBool* m_dump_xfb_target;
   GraphicsBool* m_disable_vram_copies;
   GraphicsBool* m_load_custom_textures;