[legal] Add FILE_LICENCE for core/errno.c

Signed-off-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/errno.c

 #include <errno.h>
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /** @file
  *
  * Error codes