sfc: Select PTP_1588_CLOCK

This was missed in commit a24006ed
('ptp: Enable clock drivers along with associated net/PHY drivers')
which enabled sfc's clock driver unconditionally.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/Kconfig

@@ -5,6 +5,7 @@ config SFC
 	select CRC32
 	select I2C
 	select I2C_ALGOBIT
+	select PTP_1588_CLOCK
 	---help---
 	  This driver supports 10-gigabit Ethernet cards based on
 	  the Solarflare SFC4000 and SFC9000-family controllers.