[VPlan] Add destructor to VPReductionRecipe to unbreak build.

Attempt to unbreak
https://lab.llvm.org/buildbot/#/builders/67/builds/3363/steps/6/logs/stdio

lib/Transforms/Vectorize/VPlan.h

@@ -1144,6 +1144,8 @@ public:
     addOperand(&Start);
   }
 
+  ~VPReductionPHIRecipe() override = default;
+
   /// Method to support type inquiry through isa, cast, and dyn_cast.
   static inline bool classof(const VPRecipeBase *R) {
     return R->getVPDefID() == VPRecipeBase::VPReductionPHISC;