Merge: gitignore: ignore nit_compile folders and contents

nit_compile are a series of generated intermediate C files that should
not be versioned.

We therefore add them to the .gitignore to eliminate them from the
index.

Signed-off-by: Lucas Bajolet <lucas.bajolet@gmail.com>

Pull-Request: #2627
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>