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: NShawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/Kconfig

@@ -6,6 +6,7 @@ config MLX4_EN
 	tristate "Mellanox Technologies 10Gbit Ethernet support"
 	depends on PCI
 	select MLX4_CORE
+	select PTP_1588_CLOCK
 	---help---
 	  This driver supports Mellanox Technologies ConnectX Ethernet
 	  devices.