[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/ops_inode.h b/fs/gfs2/ops_inode.h
index 6f4b547..b15acb4 100644
--- a/fs/gfs2/ops_inode.h
+++ b/fs/gfs2/ops_inode.h
@@ -10,6 +10,8 @@
 #ifndef __OPS_INODE_DOT_H__
 #define __OPS_INODE_DOT_H__
 
+#include <linux/fs.h>
+
 extern struct inode_operations gfs2_file_iops;
 extern struct inode_operations gfs2_dir_iops;
 extern struct inode_operations gfs2_symlink_iops;