[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/lm.h b/fs/gfs2/lm.h
index dbef888..6b890e7 100644
--- a/fs/gfs2/lm.h
+++ b/fs/gfs2/lm.h
@@ -10,6 +10,8 @@
 #ifndef __LM_DOT_H__
 #define __LM_DOT_H__
 
+struct gfs2_sbd;
+
 #define GFS2_MIN_LVB_SIZE 32
 
 int gfs2_lm_mount(struct gfs2_sbd *sdp, int silent);