MAINTAINERS: Add entry for CAKE qdisc

We would like the existing community to be kept in the loop for any new
developments on CAKE; and I certainly plan to keep maintaining it. Reflect
this in MAINTAINERS.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>