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

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

Fixes: 6b34a09 ("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

arch/powerpc/mm/book3s64/hash_utils.c

@@ -1012,7 +1012,7 @@ static void __init hash_init_partition_table(phys_addr_t hash_table,
 
 void hpt_clear_stress(void);
 static struct timer_list stress_hpt_timer;
-void stress_hpt_timer_fn(struct timer_list *timer)
+static void stress_hpt_timer_fn(struct timer_list *timer)
 {
 	int next_cpu;