MAINTAINERS: kunit: add David Gow as a maintainer of KUnit

David has been a de facto maintainer of KUnit for a long time now.
Formalize this in the MAINTAINERS file.

Link: https://lkml.kernel.org/r/20220725220737.790976-1-brendan.higgins@linux.dev


Signed-off-by: Brendan Higgins <brendan.higgins@linux.dev>
Reviewed-by: David Gow <davidgow@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>