openvswitch: fix compilation when vxlan is a module

With CONFIG_VXLAN=m and CONFIG_OPENVSWITCH=y, there was the following
compilation error:
  LD      init/built-in.o
  net/built-in.o: In function `vxlan_tnl_create':
  .../net/openvswitch/vport-netdev.c:322: undefined reference to `vxlan_dev_create'
  make: *** [vmlinux] Error 1

CC: Thomas Graf <tgraf@suug.ch>
Fixes: 614732ea ("openvswitch: Use regular VXLAN net_device device")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>