MAINTAINERS: Resume MPTCP co-maintainer role

I'm returning to the MPTCP maintainer role I held for most of the
subsytem's history. This time I'm using my kernel.org email address.

Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/mptcp/af85e467-8d0a-4eba-b5f8-e2f2c5d24984@tessares.net/


Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230418231318.115331-1-martineau@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>