Create Harshad_number_check.c

[Manojkumar2806]

Implemented a C program to determine if a given number is a Harshad number.

Added the isHarshad function to calculate the sum of digits and check divisibility.

Included input handling and result display in the main function.

I am creating a pull request for...

• [x] New algorithm
• [ ] Update to an algorithm
• [ ] Fix an error
• [ ] Other - Describe below