MAINTAINERS: add Rust KUnit files to the KUnit entry

The KUnit maintainers would like to maintain these files on their
side too (thanks!), so add them to their entry.

With this in place, `scripts/get_maintainer.pl` prints both sets
of maintainers/reviewers (i.e. KUnit and Rust) for those files,
which is the behavior we are looking for.

Reviewed-by: David Gow <[e-mail]>
Reviewed-by: Martin Rodriguez Reboredo <[e-mail]>
Reviewed-by: Vincenzo Palazzo <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

MAINTAINERS

···
 F:	Documentation/dev-tools/kunit/
 F:	include/kunit/
 F:	lib/kunit/
+F:	rust/kernel/kunit.rs
+F:	scripts/rustdoc_test_*
 F:	tools/testing/kunit/
 
 KERNEL USERMODE HELPER