Fix bad override

apps/openmw/mwworld/inventorystore.cpp

@@ -623,7 +623,7 @@ void MWWorld::InventoryStore::flagAsModified()
     mRechargingItemsUpToDate = false;
 }
 
-bool MWWorld::InventoryStore::stacks(const ConstPtr& ptr1, const ConstPtr& ptr2)
+bool MWWorld::InventoryStore::stacks(const ConstPtr& ptr1, const ConstPtr& ptr2) const
 {
     bool canStack = MWWorld::ContainerStore::stacks(ptr1, ptr2);
     if (!canStack)

apps/openmw/mwworld/inventorystore.hpp

@@ -174,7 +174,7 @@ namespace MWWorld
             ///< \attention This function is internal to the world model and should not be called from
             /// outside.
 
-            virtual bool stacks (const ConstPtr& ptr1, const ConstPtr& ptr2);
+            virtual bool stacks (const ConstPtr& ptr1, const ConstPtr& ptr2) const;
             ///< @return true if the two specified objects can stack with each other
 
             virtual int remove(const Ptr& item, int count, const Ptr& actor);