Complete the AddToEnd() function

Insert a node that stores data at the end of the list. The traversal of the linked list to get to the end has already been written for you.

In List.cpp, edit List::AddToEnd(int data).
Implement the pointer logic in the section of code labeled "PUT CODE HERE". For example, suppose the linked list is
```
  	m_head -> dummy -> 1 -> 4 -> 5 -> 7 -> 8 -> NULL
```
and you want to add 6. current would already be pointing to the 8: your job is to allocate a new Node from the heap, initialize it, and insert it into the list after the 8 so that the list looks like
```
  	m_head -> dummy -> 1 -> 4 -> 5 -> 7 -> 8 -> 6 -> NULL
```