MAINTAINERS: add proc sysctl KUnit test to PROC SYSCTL section

Add entry for the new proc sysctl KUnit test to the PROC SYSCTL section.

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