net: move is_vlan_dev into public header file (v2)

Migrate is_vlan_dev() to if_vlan.h so that core networkig can use it

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: davem@davemloft.net
CC: bhutchings@solarflare.com
Signed-off-by: David S. Miller <davem@davemloft.net>