MAINTAINERS: Add tools/net to NETWORKING [GENERAL]

Make sure patches for these tools go to the netdev list as well.

References: https://marc.info/?l=linux-kernel&m=139450284501328&w=2


Cc: David S. Miller <davem@davemloft.net>
Cc: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>