lustre: remove unused declaration

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/staging/lustre/lustre/llite/llite_internal.h

@@ -631,8 +631,6 @@ struct ll_file_data {
 
 struct lov_stripe_md;
 
-extern spinlock_t inode_lock;
-
 extern struct dentry *llite_root;
 extern struct kset *llite_kset;