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 <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Acked-by: Yury Norov <[e-mail]>
Signed-off-by: Yury Norov <[e-mail]>

MAINTAINERS

···
 F:	include/linux/nodemask.h
 F:	lib/bitmap.c
 F:	lib/cpumask.c
+F:	lib/cpumask_kunit.c
 F:	lib/find_bit.c
 F:	lib/find_bit_benchmark.c
 F:	lib/test_bitmap.c