MAINTAINERS: add entry for KUnit the unit testing framework

Add myself as maintainer of KUnit, the Linux kernel's unit testing
framework.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>