git: ignore nitunit.xml files

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
Merge: git: ignore nitunit.xml files

These nasty files start swarming all over my Nit folder...

Ignore!

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

Pull-Request: #929
Reviewed-by: Jean Privat <jean@pryen.org>