net/mlx5: Avoid eswitch header inclusion in fs core layer

Flow steering core layer is independent of the eswitch layer.
Hence avoid fs_core dependency on eswitch.

Fixes: 328edb49

 ("net/mlx5: Split FDB fast path prio to multiple namespaces")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>