time/sched_clock: Export symbol for sched_clock register function

time/sched_clock: Export symbol for sched_clock register function

The timer drivers could be converted into modules. The different
functions to register the clocksource or the clockevent are already
exporting their symbols for modules but the sched_clock_register()
function is missing.

Export the symbols so the drivers using this function can be converted
into modules.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Will McVicker <willmcvicker@google.com>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/r/20250602151853.1942521-8-daniel.lezcano@linaro.org