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>

tools/lib/lockdep/uinclude/linux/rcu.h

@@ -13,4 +13,9 @@ static inline int rcu_is_cpu_idle(void)
 	return 1;
 }
 
+static inline bool rcu_is_watching(void)
+{
+	return false;
+}
+
 #endif