Mark some destructors that are overriden as `override`

Summary: This satisfies a new error in llvm11

Reviewed By: smeenai

Differential Revision: D27421459

fbshipit-source-id: 963d809a4f430a365223786ea7d2591fe239062f

folly/executors/SequencedExecutor.h

@@ -26,7 +26,7 @@ namespace folly {
 // (i.e. B() can be called only after A() returns) too.
 class SequencedExecutor : public virtual Executor {
  public:
-  virtual ~SequencedExecutor() {}
+  virtual ~SequencedExecutor() override {}
 };
 
 } // namespace folly