mm/page_alloc: Split per cpu page lists and zone stats -fix -fix

mm/ is not W=1 clean for make allnoconfig but this patch accidentally made it worse for i386 with the warning

mm/vmstat.c: In function ‘refresh_cpu_vm_stats’:
mm/vmstat.c:785:34: warning: unused variable ‘pcp’ [-Wunused-variable]

     struct per_cpu_pages __percpu *pcp = zone->per_cpu_pageset;
                                    ^~~

This is a second fix to the mmotm patch mm-page_alloc-split-per-cpu-page-lists-and-zone-stats.patch.

---
 mm/vmstat.c | 2 ++
 1 file changed, 2 insertions(+)

index a2c3f58253be..f1400ba46beb 100644
@@ -782,7 +782,9 @@ static int refresh_cpu_vm_stats(bool do_pagesets)
 
 	for_each_populated_zone(zone) {
 		struct per_cpu_zonestat __percpu *pzstats = zone->per_cpu_zonestats;
+#ifdef CONFIG_NUMA
 		struct per_cpu_pages __percpu *pcp = zone->per_cpu_pageset;
+#endif
 
 		for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
 			int v;