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: 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>