selftests/tc-testing: add ptp_mock Kconfig dependency

For offloaded tc-taprio testing with netdevsim, the mock-up PHC driver
is used.

Suggested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20230807193324.4128292-10-vladimir.oltean@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>