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>