net/mlx5e: Fix tc_tun.h to verify MLX5_ESWITCH config

Exclude contents of tc_tun.h header when CONFIG_MLX5_ESWITCH is disabled to
prevent compile-time errors when compiling with such config.

Fixes: 0d9f9647

 ("net/mlx5e: Extract tc tunnel encap/decap code to dedicated file")
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>