MAINTAINERS: add Ondrej Mosnacek as a SELinux reviewer

Add Ondrej as a trusted SELinux reviewer.  Ondrej has a long history
of providing quality SELinux kernel patches and we're lucky to have
him as an official SELinux reviewer.

Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Acked-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>