Add a libft.h file check

[sjatkins12]

The moulinette requires a libft.h file to be located at the root of the repo. A simple check during the required functions would cover it.

[Seluj78]

This is already checked but it checks for the file, not whether or not the file is on the root folder or not.

This is done (from my understanding) here.

From what I get it might be done on purpose so that people can use 42FC after they validated the project when their files are in separate folders (See mine for example)