mlxsw: spectrum_span: Replace zero-length array with flexible-array member

In a similar fashion to commit e99f8e7f ("mlxsw: Replace zero-length
array with flexible-array member"), use a flexible-array member to get a
compiler warning in case the flexible array does not occur last in the
structure.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlxsw/spectrum_span.c

@@ -22,7 +22,7 @@ struct mlxsw_sp_span {
 	struct mlxsw_sp *mlxsw_sp;
 	atomic_t active_entries_count;
 	int entries_count;
-	struct mlxsw_sp_span_entry entries[0];
+	struct mlxsw_sp_span_entry entries[];
 };
 
 static void mlxsw_sp_span_respin_work(struct work_struct *work);