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 <[e-mail]>
Signed-off-by: Vladimir Oltean <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/tc-testing/config

···
 CONFIG_NET_SCH_ETS=m
 CONFIG_NET_SCH_RED=m
 CONFIG_NET_SCH_FQ_PIE=m
-CONFIG_NETDEVSIM=m
 
 #
 ## Network testing
 #
 CONFIG_CAN=m
 CONFIG_ATM=y
+CONFIG_NETDEVSIM=m
+CONFIG_PTP_1588_CLOCK_MOCK=m