NOTE: Pass by value only.
Assume that your program has a main() function, an enterAge() function, and a birthday() function.
The main() method will call the enterAge() with no parameters; the enterAge() will return the age entered by the user.
The main() method then passes the age to the birthday() function as the only parameter. The birthday() function increments the age by one, and returns the incremented age to the main() function via the return statement.
TODO: Code the prototype for the enterAge() function as it would appear in the main() function.