net/mlx5: make the symbol 'ESW_POOLS' static

Fix the following sparse warning:
drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads_chains.c:35:20: warning: symbol 'ESW_POOLS' was not declared. Should it be static?

Fixes: 39ac237c ("net/mlx5: E-Switch, Refactor chains and priorities")
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Acked-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>