powerpc/64s/hash: Make stress_hpt_timer_fn() static

stress_hpt_timer_fn() is only used in hash_utils.c, make it static.

Fixes: 6b34a099 ("powerpc/64s/hash: add stress_hpt kernel boot option to increase hash faults")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221228093603.3166599-1-yangyingliang@huawei.com