LibDraw: Override set_{non,}volatile in GIFLoader

Author: https://github.com/jcs
Commit: https://github.com/SerenityOS/serenity/commit/0c20f98c1c7
Pull-request: https://github.com/SerenityOS/serenity/pull/1177

Libraries/LibDraw/GIFLoader.h

@@ -41,6 +41,8 @@ public:
 
     virtual Size size() override;
     virtual RefPtr<GraphicsBitmap> bitmap() override;
+    virtual void set_volatile() override;
+    [[nodiscard]] virtual bool set_nonvolatile() override;
 
 private:
     OwnPtr<GIFLoadingContext> m_context;