8021q: make vlan_pcpu_stats visible without CONFIG_VLAN_8021Q

macvlan needs vlan_pcpu_stats so make it visible even if compiling
without VLAN_8021Q support. Otherwise a very long compiler error happens.

Fixes: cdf3e274 ("macvlan: unify macvlan_pcpu_stats and vlan_pcpu_stats")
Cc: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-By: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>