sched: Hide unused sched_update_scaling()

This function is only used when CONFIG_SMP is enabled, without that there
is no caller and no prototype:

kernel/sched/fair.c:688:5: error: no previous prototype for 'sched_update_scaling' [-Werror=missing-prototypes

Hide the definition in the same #ifdef check as the declaration.

Fixes: 8a99b683

 ("sched: Move SCHED_DEBUG sysctl to debugfs")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230522195021.3456768-2-arnd@kernel.org