[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/rgrp.h b/fs/gfs2/rgrp.h
index 3d6ae37..3558957 100644
--- a/fs/gfs2/rgrp.h
+++ b/fs/gfs2/rgrp.h
@@ -10,6 +10,10 @@
 #ifndef __RGRP_DOT_H__
 #define __RGRP_DOT_H__
 
+struct gfs2_rgrpd;
+struct gfs2_sbd;
+struct gfs2_holder;
+
 void gfs2_rgrp_verify(struct gfs2_rgrpd *rgd);
 
 struct gfs2_rgrpd *gfs2_blk2rgrpd(struct gfs2_sbd *sdp, u64 blk);