Fix ObjectList inheritance

[mikee47]

This PR fixes a bug where inheriting from a base LinkedObjectListTemplate implementation does not work as expected.

By example, consider these class definitions:

#include <Data/LinkedObjectList.h>

class BaseObject : public LinkedObjectTemplate<BaseObject>
{
public:
	using List = LinkedObjectListTemplate<BaseObject>;
	using OwnedList = OwnedLinkedObjectListTemplate<BaseObject>;
};

class ObjectOne : public BaseObject
{
public:
	using List = LinkedObjectListTemplate<ObjectOne>;
	using OwnedList = OwnedLinkedObjectListTemplate<ObjectOne>;

public:
	ObjectOne(int value) : prop1(value)
	{
	}

	int prop1;
};

void test()
{
    ObjectOne::OwnedList list;
    for(auto obj : list) {
        Serial << obj.prop1 << " ";
    }
}

This fails to compile with the error ‘class {anonymous}::BaseObject’ has no member named ‘prop1’,
because the iterator returns BaseObject instead of ObjectOne, which is incorrect behaviour.