Explicitly use correct flags type in Executor::makeKeepAlive

Summary: [Folly] Explicitly use correct flags type in `Executor::makeKeepAlive` rather than relying on implicit conversion from `bool` after {D15683241}.

Reviewed By: andriigrynenko

Differential Revision: D15835078

fbshipit-source-id: 49d7895bc909996b646c19c6a2fa0690a58cbe13

folly/Executor.h

@@ -230,7 +230,7 @@ class Executor {
     static_assert(
         std::is_base_of<Executor, ExecutorT>::value,
         "makeKeepAlive only works for folly::Executor implementations.");
-    return KeepAlive<ExecutorT>{executor, false};
+    return KeepAlive<ExecutorT>{executor, uintptr_t(0)};
   }
 
  private: