vmstat: include compaction.h when CONFIG_COMPACTION

This removes following warning from sparse:

 mm/vmstat.c:466:5: warning: symbol 'fragmentation_index' was not declared. Should it be static?

[akpm@linux-foundation.org: move the include to top-of-file]
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmstat.c

@@ -18,6 +18,7 @@
 #include <linux/sched.h>
 #include <linux/math64.h>
 #include <linux/writeback.h>
+#include <linux/compaction.h>
 
 #ifdef CONFIG_VM_EVENT_COUNTERS
 DEFINE_PER_CPU(struct vm_event_state, vm_event_states) = {{0}};
@@ -395,6 +396,7 @@ void zone_statistics(struct zone *preferred_zone, struct zone *z)
 #endif
 
 #ifdef CONFIG_COMPACTION
+
 struct contig_page_info {
 	unsigned long free_pages;
 	unsigned long free_blocks_total;