folly/executors/ManualExecutor.h: mark overriding functions with "override"

Summary:
This avoids the following errors:

  folly/executors/ManualExecutor.h:42:3: error: '~ManualExecutor' overrides a destructor but is not marked 'override' [-Werror,-Wsuggest-destructor-override]

Reviewed By: r-barnes

Differential Revision: D33397861

fbshipit-source-id: f9bee89aeaa55d365155dc3fdc0b70b283980a2a

folly/executors/ManualExecutor.h

@@ -39,7 +39,7 @@ class ManualExecutor : public DrivableExecutor,
                        public ScheduledExecutor,
                        public SequencedExecutor {
  public:
-  ~ManualExecutor();
+  ~ManualExecutor() override;
 
   void add(Func) override;