[GFS2] Make headers compile on their own

As per Jan Engelhardt's comments, this should make all the headers
compile on their own by including and/or declaring structures
early.

Cc: Jan Engelhardt <jengelh@linux01.gwdg.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/log.h b/fs/gfs2/log.h
index 80764e3..7f5737d 100644
--- a/fs/gfs2/log.h
+++ b/fs/gfs2/log.h
@@ -10,6 +10,10 @@
 #ifndef __LOG_DOT_H__
 #define __LOG_DOT_H__
 
+#include <linux/list.h>
+#include <linux/spinlock.h>
+#include "incore.h"
+
 /**
  * gfs2_log_lock - acquire the right to mess with the log manager
  * @sdp: the filesystem