MAINTAINERS: add git trees for MPTCP

This will help occasional developers to find our git repo without having
to look at our wiki.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>