MAINTAINERS: add proc sysctl KUnit test to PROC SYSCTL section

Add entry for the new proc sysctl KUnit test to the PROC SYSCTL section,
and add Iurii as a maintainer.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Cc: Iurii Zaikin <yzaikin@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>