IB/ipath: Depend on CONFIG_NET

ipath uses skb functions and won't build without CONFIG_NET.

Spotted by Randy Dunlap.

Signed-off-by: Bryan O'Sullivan <bryan.osullivan@qlogic.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>