net/mlx5: Fix compilation warning in eq.c

mlx5_eq_table_get_rmap is being used only when CONFIG_RFS_ACCEL is
enabled, this patch fixes the below warning when CONFIG_RFS_ACCEL is
disabled.

drivers/.../mlx5/core/eq.c:903:18: [-Werror=missing-prototypes]
error: no previous prototype for ‘mlx5_eq_table_get_rmap’

Fixes: f2f3df55 ("net/mlx5: EQ, Privatize eq_table and friends")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>