[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/mount.h b/fs/gfs2/mount.h
index 8a21897..401288a 100644
--- a/fs/gfs2/mount.h
+++ b/fs/gfs2/mount.h
@@ -10,6 +10,8 @@
 #ifndef __MOUNT_DOT_H__
 #define __MOUNT_DOT_H__
 
+struct gfs2_sbd;
+
 int gfs2_mount_args(struct gfs2_sbd *sdp, char *data_arg, int remount);
 
 #endif /* __MOUNT_DOT_H__ */