src: mark AsyncWrap destructor as virtual

Make AsyncWrap::~AsyncWrap() virtual, otherwise it is unsafe to delete
a derived class through a pointer to AsyncWrap.  Fortunately, we don't
do that anywhere right now; this commit is merely a preemptive bug fix.

src/async-wrap.h

@@ -61,7 +61,7 @@ class AsyncWrap : public BaseObject {
                    v8::Handle<v8::Object> object,
                    ProviderType provider);
 
-  inline ~AsyncWrap() = default;
+  inline virtual ~AsyncWrap() = default;
 
   inline bool has_async_listener();