[SNMP]: Add definitions for {In,Out}BcastPkts

The updated IP-MIB RFC (RFC4293) specifys new objects, InBcastPkts
and OutBcastPkts. This adds definitions for them.

Signed-off-by: Mitsuru Chinen <mitch@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>