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>