selftests/timens: timerfd: Use correct clockid type in tclock_gettime()

tclock_gettime() is a wrapper around clock_gettime().
The first parameter of clock_gettime() is of type "clockid_t",
not "clock_t".

Use the correct type instead.

Link: https://lore.kernel.org/r/20250502-selftests-timens-fixes-v1-3-fb517c76f04d@linutronix.de


Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>