Revert "i40e: remove CONFIG_I40E_VXLAN"

This reverts commit 8fe26999

.
The case where VXLAN is a module and i40e driver is inbuilt
will not be handled properly with this change since i40e
will have an undefined symbol vxlan_get_rx_port in it.

v2: Add a signed-off-by.

Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>