aufs: tiny, missing a parameter declaration

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit 9dbd45984201453ecebcc03a91699878aa38239d)

fs/aufs/loop.h

@@ -27,7 +27,7 @@ void au_warn_loopback(struct super_block *h_sb);
 int au_loopback_init(void);
 void au_loopback_fin(void);
 #else
-AuStub(struct file *, loop_backing_file, return NULL)
+AuStub(struct file *, loop_backing_file, return NULL, struct super_block *sb)
 
 AuStubInt0(au_test_loopback_overlap, struct super_block *sb,
 	   struct dentry *h_adding)