mm: make scan_all_zones_unevictable_pages() static

sparse output following warning.

	mm/vmscan.c:2549:6: warning: symbol 'scan_all_zones_unevictable_pages' was not declared. Should it be static?

cleanup here.
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2506,7 +2506,7 @@ void scan_zone_unevictable_pages(struct zone *zone)
  * that has possibly/probably made some previously unevictable pages
  * evictable.
  */
-void scan_all_zones_unevictable_pages(void)
+static void scan_all_zones_unevictable_pages(void)
 {
 	struct zone *zone;