MAINTAINERS: Add source tree entry for kunit

Patches for kunit are managed in kunit and kunit-fixes branches of
linux-kselftest tree before merged into the mainline.  However, the
MAINTAINERS section for kunit is not having the entries for the
branches.  Add the entries.

Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>