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.comSigned-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>

include/linux/compaction.h

@@ -226,8 +226,8 @@ static inline void wakeup_kcompactd(pg_data_t *pgdat, int order, int classzone_i
 
 #endif /* CONFIG_COMPACTION */
 
-#if defined(CONFIG_COMPACTION) && defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
 struct node;
+#if defined(CONFIG_COMPACTION) && defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
 extern int compaction_register_node(struct node *node);
 extern void compaction_unregister_node(struct node *node);