net/mlx5e: vxlan.c depends on CONFIG_VXLAN

When vxlan is not enabled by kernel, no need to enable it in mlx5.
Compile out lib/vxlan.c if CONFIG_VXLAN is not selected.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>