net: ptp: do not use a reserved id in global scope

Generally, we should avoid using reserved names in global
contexts.

subsys/net/lib/ptp/clock.c:58:25: \
  error: 'clock' redeclared as different kind of symbol
   58 | static struct ptp_clock clock = { 0 };

/opt/toolchains/zephyr-sdk-0.16.8/arm-zephyr-eabi/picolibc/ \
  include/time.h:100:12: \
  note: previous declaration of 'clock' with type 'clock_t(void)' \
  {aka 'long unsigned int(void)'}
  100 | clock_t    clock (void);

subsys/net/lib/ptp/clock.c:58:25: \
  error: 'clock' defined but not used [-Werror=unused-variable]
   58 | static struct ptp_clock clock = { 0 };

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>