Minor change to fix build on Xcode 10 (google#2523)

Author: Qining

gapii/cc/spy_base.h

@@ -53,6 +53,7 @@ class PackEncoder;
 class SpyBase {
  public:
   SpyBase();
+  virtual ~SpyBase() {}
 
   void init(CallObserver* observer);
 

gapir/cc/context.h

@@ -53,7 +53,7 @@ class Context : private Renderer::Listener {
                                          ResourceLoader* resource_loader,
                                          MemoryManager* memory_manager);
 
-  ~Context();
+  virtual ~Context();
 
   void prefetch(ResourceCache* cache) const;
 

gapir/cc/renderer.h

@@ -30,6 +30,7 @@ class Renderer {
    public:
     virtual void onDebugMessage(uint32_t severity, uint8_t api_index,
                                 const char* msg) = 0;
+    virtual ~Listener() {}
   };
 
   inline Renderer();