cxgb3: fix build error when INET=n

cxgb3 uses lro_* functions and selects INET_LRO, but this doesn't help unless
INET is already enabled, so make the driver depend on INET also.

sge.c:(.text+0x9f09a): undefined reference to `lro_flush_all'
sge.c:(.text+0x9f62f): undefined reference to `lro_receive_skb'
sge.c:(.text+0x9f8a3): undefined reference to `lro_receive_frags'
sge.c:(.text+0x9fbe0): undefined reference to `lro_vlan_hwaccel_receive_skb'
sge.c:(.text+0x9ffcd): undefined reference to `lro_vlan_hwaccel_receive_frags'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>