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>

net/wireless/nl80211.c

@@ -20,6 +20,7 @@
 #include <linux/netlink.h>
 #include <linux/nospec.h>
 #include <linux/etherdevice.h>
+#include <linux/if_vlan.h>
 #include <net/net_namespace.h>
 #include <net/genetlink.h>
 #include <net/cfg80211.h>