net/mlx5: add stubs on Windows

net/mlx5: add stubs on Windows

This commits adds ethdev stubs. These APIs are called from shared code
that must compile under Linux and Windows. The following stubs are added:
mlx5_set_mtu
mlx5_os_read_dev_counters
mlx5_intr_callback_unregister
mlx5_os_get_stats_n
mlx5_os_stats_init
mlx5_set_link_down
mlx5_set_link_up
mlx5_dev_get_flow_ctrl
mlx5_dev_set_flow_ctrl
mlx5_get_module_info
mlx5_get_module_eeprom

Signed-off-by: Ophir Munk <ophirmu@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>