mm: make scan_zone_unevictable_pages() static

sparse output following warning

	mm/vmscan.c:2507:6: warning: symbol 'scan_zone_unevictable_pages' was not declared. Should it be static?

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

mm/vmscan.c

@@ -2464,7 +2464,7 @@ void scan_mapping_unevictable_pages(struct address_space *mapping)
  * back onto @zone's unevictable list.
  */
 #define SCAN_UNEVICTABLE_BATCH_SIZE 16UL /* arbitrary lock hold batch size */
-void scan_zone_unevictable_pages(struct zone *zone)
+static void scan_zone_unevictable_pages(struct zone *zone)
 {
 	struct list_head *l_unevictable = &zone->lru[LRU_UNEVICTABLE].list;
 	unsigned long scan;