sfc: Select PTP_1588_CLOCK

This was missed in commit a24006ed12616bde1bbdb26868495906a212d8dc
('ptp: Enable clock drivers along with associated net/PHY drivers')
which enabled sfc's clock driver unconditionally.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>