[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/quota.h b/fs/gfs2/quota.h
index d93c61c..a8be141 100644
--- a/fs/gfs2/quota.h
+++ b/fs/gfs2/quota.h
@@ -10,6 +10,9 @@
 #ifndef __QUOTA_DOT_H__
 #define __QUOTA_DOT_H__
 
+struct gfs2_inode;
+struct gfs2_sbd;
+
 #define NO_QUOTA_CHANGE ((u32)-1)
 
 int gfs2_quota_hold(struct gfs2_inode *ip, u32 uid, u32 gid);