net/mlx5: Add missing inline to stub esw_add_restore_rule

When CONFIG_MLX5_ESWITCH is unset, clang warns:

In file included from drivers/net/ethernet/mellanox/mlx5/core/main.c:58:
drivers/net/ethernet/mellanox/mlx5/core/eswitch.h:670:1: warning: unused
function 'esw_add_restore_rule' [-Wunused-function]
esw_add_restore_rule(struct mlx5_eswitch *esw, u32 tag)
^
1 warning generated.

This stub function is missing inline; add it to suppress the warning.

Fixes: 11b717d6

 ("net/mlx5: E-Switch, Get reg_c0 value on CQE")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>