vlan: correct the file path in vlan_dev_change_flags() comment

The vlan_flags enum is defined in include/uapi/linux/if_vlan.h file.
not in include/linux/if_vlan.h file.

Signed-off-by: Sun Lianwen <sunlw.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>