[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/meta_io.h b/fs/gfs2/meta_io.h
index 9a9acc9..086a472df 100644
--- a/fs/gfs2/meta_io.h
+++ b/fs/gfs2/meta_io.h
@@ -10,6 +10,10 @@
 #ifndef __DIO_DOT_H__
 #define __DIO_DOT_H__
 
+#include <linux/buffer_head.h>
+#include <linux/string.h>
+#include "incore.h"
+
 static inline void gfs2_buffer_clear(struct buffer_head *bh)
 {
 	memset(bh->b_data, 0, bh->b_size);