Explicitly specify inheritance mode.

td/telegram/Requests.h

@@ -87,7 +87,7 @@ class Requests {
   template <class T>
   class RequestPromise
       : public PromiseInterface<T>
-      , RequestPromiseBase {
+      , private RequestPromiseBase {
    public:
     void set_value(T &&value) override {
       RequestPromiseBase::set_value(std::move(value));