mm: make init_section_page_cgroup() static

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: 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/page_cgroup.c

@@ -101,7 +101,7 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
 }
 
 /* __alloc_bootmem...() is protected by !slab_available() */
-int __init_refok init_section_page_cgroup(unsigned long pfn)
+static int __init_refok init_section_page_cgroup(unsigned long pfn)
 {
 	struct mem_section *section;
 	struct page_cgroup *base, *pc;