net: lan966x: Fix compilation error

Starting from the blamed commit, the lan966x build fails with the
following compilation error:

drivers/net/ethernet/microchip/lan966x/lan966x_ptp.c:342:9: error: implicit declaration of function ‘ptp_find_pin_unlocked’ [-Werror=implicit-function-declaration]
  342 |   pin = ptp_find_pin_unlocked(phc->clock, PTP_PF_EXTTS, 0);

The issue is that there is no stub function for ptp_find_pin_unlocked
in case CONFIG_PTP_1588_CLOCK is not selected. Therefore add one.
Reported-by: kernel test robot <lkp@intel.com>
Fixes: f3d8e0a9 ("net: lan966x: Add support for PTP_PF_EXTTS")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ptp_clock_kernel.h

@@ -321,6 +321,10 @@ static inline int ptp_clock_index(struct ptp_clock *ptp)
 static inline int ptp_find_pin(struct ptp_clock *ptp,
 			       enum ptp_pin_function func, unsigned int chan)
 { return -1; }
+static inline int ptp_find_pin_unlocked(struct ptp_clock *ptp,
+					enum ptp_pin_function func,
+					unsigned int chan)
+{ return -1; }
 static inline int ptp_schedule_worker(struct ptp_clock *ptp,
 				      unsigned long delay)
 { return -EOPNOTSUPP; }