UndelayedDestruction virtual=>override

Summary:
Had this in another diff, but it caused too much test noise. So I've separated
it out on it's own.

Reviewed By: yfeldblum

Differential Revision: D3213605

fb-gh-sync-id: 9ebfcf8430da7c66a31868032a0cef1e616ffc58
fbshipit-source-id: 9ebfcf8430da7c66a31868032a0cef1e616ffc58

folly/io/async/test/UndelayedDestruction.h

@@ -96,7 +96,7 @@ class UndelayedDestruction : public TDD {
    * Override our parent's destroy() method to make it protected.
    * Callers should use the normal destructor instead of destroy
    */
-  virtual void destroy() {
+  void destroy() override {
     this->TDD::destroy();
   }