net: sctp: trivial: add uapi/linux/sctp.h into maintainers

After this file has moved to the uapi section, we also need to update
this in the maintainers file.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>