net: core: fix compile error inside flow_dissector due to new dsa callback

The following error was introduced by
commit 43e66528

 ("net-next: dsa: fix flow dissection")
due to a missing #if guard

net/core/flow_dissector.c: In function '__skb_flow_dissect':
net/core/flow_dissector.c:448:18: error: 'struct net_device' has no member named 'dsa_ptr'
ops = skb->dev->dsa_ptr->tag_ops;
                ^
make[3]: *** [net/core/flow_dissector.o] Error 1

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>