Bug 1057908 followup: Add missing 'override' annotation on GMPServiceParent::ActorDestroy method decl. rs=ehsan

dom/media/gmp/GMPServiceParent.h

@@ -197,7 +197,7 @@ public:
                                             bool* aHasPlugin,
                                             nsCString* aVersion);
 
-  virtual void ActorDestroy(ActorDestroyReason aWhy);
+  virtual void ActorDestroy(ActorDestroyReason aWhy) override;
 
   static PGMPServiceParent* Create(Transport* aTransport, ProcessId aOtherPid);