net/mlx5e: Fix build error without IPV6

If IPV6 is not set and CONFIG_MLX5_ESWITCH is y,
building fails:

drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun.c:322:5: error: redefinition of mlx5e_tc_tun_create_header_ipv6
 int mlx5e_tc_tun_create_header_ipv6(struct mlx5e_priv *priv,
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun.c:7:0:
drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun.h:67:1: note: previous definition of mlx5e_tc_tun_create_header_ipv6 was here
 mlx5e_tc_tun_create_header_ipv6(struct mlx5e_priv *priv,
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use #ifdef to guard this, also move mlx5e_route_lookup_ipv6
to cleanup unused warning.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: e689e998 ("net/mlx5e: TC, Stub out ipv6 tun create header function")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>