IB/mlx5: Declare devx_async_cmd_event_fops static

Avoid that sparse complains about a missing declaration.

Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Fixes: 6bf8f22a

 ("IB/mlx5: Introduce MLX5_IB_OBJECT_DEVX_ASYNC_CMD_FD")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>