Fix a minor fallout from the merge of the timers and the networking
trees. The following error may result if the PTP_1588_CLOCK
prerequisites are not available:

drivers/built-in.o: In function `ptp_clock_unregister':
(.text+0x40e0a5): undefined reference to `pps_unregister_source'
drivers/built-in.o: In function `ptp_clock_unregister':
(.text+0x40e0cc): undefined reference to `posix_clock_unregister'
drivers/built-in.o: In function `ptp_clock_event':
(.text+0x40e249): undefined reference to `pps_event'
drivers/built-in.o: In function `ptp_clock_register':
(.text+0x40e5e1): undefined reference to `pps_register_source'
drivers/built-in.o: In function `ptp_clock_register':
(.text+0x40e62c): undefined reference to `posix_clock_register'
drivers/built-in.o: In function `ptp_clock_register':
(.text+0x40e68d): undefined reference to `pps_unregister_source'

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>