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>

Author: mjmartineau

MAINTAINERS

@@ -14594,6 +14594,7 @@ F:	net/netlabel/
 
 NETWORKING [MPTCP]
 M:	Matthieu Baerts <matthieu.baerts@tessares.net>
+M:	Mat Martineau <martineau@kernel.org>
 L:	netdev@vger.kernel.org
 L:	mptcp@lists.linux.dev
 S:	Maintained