sched: add Meng as RTDS maintainer

sched: add Meng as RTDS maintainer

Meng Xu is one of the maintainers of the RT-Xen project,
which is from where the RTDS scheduler comes. He also
is the main author of the version of RTDS that we currently
have here upstream.

Since the upstreaming effort, he's continued looking after
the code, engaging with the community, coming and presenting
at XenSummits and, last but not least, doing development
himself as well as directing the work of others, with the
aim of improving the scheduler.

In summary, he has reached the point, both from thechnical
and community engagement point of views, where he can
effectively serve as a maintainer of RTDS code.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Meng Xu <mengxu@cis.upenn.edu>
Acked-by: George Dunlap <george.dunlap@citrix.com>