[SCTP]: Add SENTINEL to SCTP MIB stats

Add SNMP_MIB_SENTINEL to the definition of the sctp_snmp_list so that
the output routine in proc correctly terminates.  This was causing some
problems running on ia64 systems.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>