tools/liblockdep: Add a stub for new rcu_is_watching

Stub out rcu_is_watching(), prevents build error with the updated
tree.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
diff --git a/tools/lib/lockdep/uinclude/linux/rcu.h b/tools/lib/lockdep/uinclude/linux/rcu.h
index 4c99fcb..042ee8e 100644
--- a/tools/lib/lockdep/uinclude/linux/rcu.h
+++ b/tools/lib/lockdep/uinclude/linux/rcu.h
@@ -13,4 +13,9 @@
 	return 1;
 }
 
+static inline bool rcu_is_watching(void)
+{
+	return false;
+}
+
 #endif