use override in ClientField, MenuHandler (#2665)

gframe/client_field.h

@@ -144,8 +144,8 @@ public:
 	ClientCard* menu_card{ nullptr };
 	int list_command{ 0 };
 
-	virtual bool OnEvent(const irr::SEvent& event);
-	virtual bool OnCommonEvent(const irr::SEvent& event);
+	bool OnEvent(const irr::SEvent& event) override;
+	bool OnCommonEvent(const irr::SEvent& event);
 	void GetHoverField(int x, int y);
 	void ShowMenu(int flag, int x, int y);
 	void HideMenu();

gframe/menu_handler.h

@@ -7,7 +7,7 @@ namespace ygo {
 
 class MenuHandler: public irr::IEventReceiver {
 public:
-	virtual bool OnEvent(const irr::SEvent& event);
+	bool OnEvent(const irr::SEvent& event) override;
 	s32 prev_operation;
 	int prev_sel;