ifb: fix building without CONFIG_NET_CLS_ACT

The driver no longer depends on this option, but it fails to
build if it's disabled because the skb->tc_skip_classify is
hidden behind an #ifdef:

drivers/net/ifb.c:81:8: error: no member named 'tc_skip_classify' in 'struct sk_buff'
                skb->tc_skip_classify = 1;

Use the same #ifdef around the assignment.

Fixes: 046178e7

 ("ifb: Depend on netfilter alternatively to tc")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>