tools/kernel.h: hide task_struct.hardirq_chain_key

As a result of c86e9b987cea ("lockdep: Prepare for noinstr sections") we need
to hide hardirq_chain_key from userspace to avoid a build error.

Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/tools/include/linux/lockdep.h b/tools/include/linux/lockdep.h
index 4e5f31f..b6c9e81 100644
--- a/tools/include/linux/lockdep.h
+++ b/tools/include/linux/lockdep.h
@@ -40,6 +40,7 @@
 	unsigned long hardirq_disable_ip, hardirq_enable_ip;
 	unsigned int hardirq_disable_event, hardirq_enable_event;
 	int softirq_context, hardirq_context;
+	unsigned long hardirq_chain_key;
 	char comm[17];
 };