sched/fair: Make update_entity_lag() static

The function update_entity_lag() is only used inside the kernel/sched/fair.c file.
Make it static.

Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230829030325.69128-1-jiahao.os@bytedance.com