I want to add C++ code of AVL Tree

[X] Issue Title is in form avl_tree [C++].
[X] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
[X] This Algo/DS is worth adding to this repository.
[X] Assign this issue to me (I want to work on this).
- [X] I ensure that I am not already assigned to 2 or more issues.
[X] I will send my PR only after I'm assigned to this issue by any maintainer.
[X] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

iiitv / algos