include/linux/compaction.h: fix potential build error

Declaration of struct node is required regardless.  On UMA systems,
including compaction.h without preceding node.h shouldn't cause a build
error.

Link: http://lkml.kernel.org/r/20190208080437.253322-1-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>