add constness to UICheckBox

cocos/ui/UICheckBox.cpp

@@ -343,7 +343,7 @@ void CheckBox::setSelectedState(bool selected)
     _frontCrossRenderer->setVisible(_isSelected);
 }
 
-bool CheckBox::getSelectedState()
+bool CheckBox::getSelectedState()const
 {
     return _isSelected;
 }

cocos/ui/UICheckBox.h

@@ -170,7 +170,7 @@ public:
      *
      * @return selected    true that checkbox is selected, false otherwise.
      */
-    bool getSelectedState();
+    bool getSelectedState()const;
 
     //add a call back function would called when checkbox is selected or unselected.
     CC_DEPRECATED_ATTRIBUTE void addEventListenerCheckBox(Ref* target,SEL_SelectedStateEvent selector);