[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/dir.h

@@ -10,6 +10,12 @@
 #ifndef __DIR_DOT_H__
 #define __DIR_DOT_H__
 
+#include <linux/dcache.h>
+
+struct inode;
+struct gfs2_inode;
+struct gfs2_inum;
+
 /**
  * gfs2_filldir_t - Report a directory entry to the caller of gfs2_dir_read()
  * @opaque: opaque data used by the function