folly | Fix inconsistent-missing-destructor-override warnings

Summary:
changelog: [internal]

Fixing inconsistent use of 'override' in destructors.

Reviewed By: nlutsenko

Differential Revision: D44942816

fbshipit-source-id: 8d0c143e268edd3ece28baa821a6ec51a2ec38e3

folly/executors/TimekeeperScheduledExecutor.h

@@ -61,7 +61,7 @@ class TimekeeperScheduledExecutor : public ScheduledExecutor {
       Function<std::shared_ptr<Timekeeper>()> getTimekeeper)
       : parent_(std::move(parent)), getTimekeeper_(std::move(getTimekeeper)) {}
 
-  ~TimekeeperScheduledExecutor() { DCHECK(!keepAliveCounter_); }
+  ~TimekeeperScheduledExecutor() override { DCHECK(!keepAliveCounter_); }
 
   void run(Func);