net/mlx5e: Add missing static function annotation

Compilation with W=1 produces following warning:

drivers/net/ethernet/mellanox/mlx5/core/en/monitor_stats.c:69:6:
warning: no previous prototype for _mlx5e_monitor_counter_start_ [-Wmissing-prototypes]
 void mlx5e_monitor_counter_start(struct mlx5e_priv *priv)
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~

Avoid it by declaring mlx5e_monitor_counter_start() as a static function.

Fixes: 5c7e8bbb ("net/mlx5e: Use monitor counters for update stats")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>