Fix some warnings on some compilers

ItemStack is a struct, not a class
CollisionInfo is a struct, not a class
InventoryAction is abstract and it needs a virtual destructor

src/inventorymanager.h

@@ -103,6 +103,7 @@ struct InventoryAction
 	virtual void apply(InventoryManager *mgr, ServerActiveObject *player,
 			IGameDef *gamedef) = 0;
 	virtual void clientApply(InventoryManager *mgr, IGameDef *gamedef) = 0;
+	virtual ~InventoryAction() {};
 };
 
 struct IMoveAction : public InventoryAction

src/player.h

@@ -30,7 +30,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 class Map;
 class IGameDef;
-class CollisionInfo;
+struct CollisionInfo;
 
 class Player
 {

src/scriptapi.h

@@ -30,7 +30,7 @@ class ServerActiveObject;
 class ServerRemotePlayer;
 typedef struct lua_State lua_State;
 struct LuaEntityProperties;
-class ItemStack;
+struct ItemStack;
 struct PointedThing;
 //class IGameDef;
 

src/serverobject.h

@@ -42,7 +42,7 @@ Some planning
 */
 
 class ServerEnvironment;
-class ItemStack;
+struct ItemStack;
 class Player;
 struct ToolDiggingProperties;