folly/io/async/ScopedEventBaseThread.h: mark overriding functions with "override"

Summary:
This avoids the following errors:

  folly/io/async/ScopedEventBaseThread.h:49:3: error: '~ScopedEventBaseThread' overrides a destructor but is not marked 'override' [-Werror,-Wsuggest-destructor-override]

Reviewed By: ryanthomasjohnson

Differential Revision: D33397489

fbshipit-source-id: 29210c0de0b850bdda5d2c27a5be8971fc17b40e

folly/io/async/ScopedEventBaseThread.h

@@ -46,7 +46,7 @@ class ScopedEventBaseThread : public IOExecutor, public SequencedExecutor {
       EventBase::Options eventBaseOptions,
       EventBaseManager* ebm,
       StringPiece name);
-  ~ScopedEventBaseThread();
+  ~ScopedEventBaseThread() override;
 
   EventBase* getEventBase() const { return &eb_; }