lib/cpumask_kunit: add tests file to MAINTAINERS

cpumask related files are listed under the BITMAP API section, so the
file with tests for cpumask should be added to that list.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>