Compile in binary_tree, add func prototypes.

data_structures/avl_tree/makefile
	-Compile in the `binary_tree` project, which exposes
	`BinaryTree_Tree_t` data structure that `AVL_Tree_t` will rely
	on.
	-Add necessary compiler flags, etc.

data_structures/avl_tree/src/avl_tree.h
	-Add prototypes for all necessary functions.
	-Declare `AVL_Node_t`.