nl80211: explicitly include if_vlan.h

We use that here, and do seem to get it through some recursive
include, but better include it explicitly.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200224093814.1b9c258fec67.I45ac150d4e11c72eb263abec9f1f0c7add9bef2b@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>