Create fermats_little_theorem.cpp

[bhavyammodi]

Description

Adding c++ code for Fermats Little Theorem.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines(Clean Code) of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have created a helpful and easy to understand README.md
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests/screenshots(if any) that prove my fix is effective or that my feature works