dog.h - Define a new type struct dog.
1-init_dog.c - Contains a function that initialize a variable of type struct dog.
2-print_dog.c - Prints a struct dog.
dog.h - Define a new type dog_t as a new name for the type struct dog.
4-new_dog.c - Write a function that creates a new dog.
5-free_dog.c - Frees dog.