mlx4_en: Select PTP_1588_CLOCK

Now that mlx4_en includes a PHC driver it must select PTP_1588_CLOCK.

   drivers/built-in.o: In function `mlx4_en_get_ts_info':
>> en_ethtool.c:(.text+0x391a11): undefined reference to `ptp_clock_index'
   drivers/built-in.o: In function `mlx4_en_remove_timestamp':
>> (.text+0x397913): undefined reference to `ptp_clock_unregister'
   drivers/built-in.o: In function `mlx4_en_init_timestamp':
>> (.text+0x397b20): undefined reference to `ptp_clock_register'

Fixes: ad7d4eae ("mlx4_en: Add PTP hardware clock")
Signed-off-by: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>