[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>

fs/gfs2/super.h

@@ -10,6 +10,8 @@
 #ifndef __SUPER_DOT_H__
 #define __SUPER_DOT_H__
 
+#include "incore.h"
+
 void gfs2_tune_init(struct gfs2_tune *gt);
 
 int gfs2_check_sb(struct gfs2_sbd *sdp, struct gfs2_sb *sb, int silent);