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 <[email protected]>
Link: https://lore.kernel.org/mptcp/[email protected]/
Signed-off-by: Mat Martineau <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: mjmartineau

MAINTAINERS

@@ -14594,6 +14594,7 @@ F:	net/netlabel/
 
 NETWORKING [MPTCP]
 M:	Matthieu Baerts <[email protected]>
+M:	Mat Martineau <[email protected]>
 L:	[email protected]
 L:	[email protected]
 S:	Maintained