Add virtual dtor to RefCount base class.

The code generated for types w/ &refcount will subclass RefCount and
Unref definitely deletes via a pointer to that base class so it needs a
virtual dtor.

lib/binpac.h.in

@@ -150,6 +150,7 @@ class RefCount
 {
 public:
 	RefCount() 	{ count = 1; }
+	virtual ~RefCount() {}
 	void Ref() 	{ ++count; }
 	int Unref() 	{ BINPAC_ASSERT(count > 0); return --count; }