vlan: Fix build error wth vlan_get_encap_level()

The new function vlan_get_encap_level() uses vlan_dev_priv()
which is only conditionally avaialble when VLAN support is
enabled.  Make vlan_get_encap_level() conditionally available
as well.

Fixes: 44a40855 ("bonding: Fix stacked device detection in arp monitoring")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
CC: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>