Sparse output following warning.

mm/page_cgroup.c:100:15: warning: symbol 'init_section_page_cgroup' 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>