Fix: compile error when destructor is private

Defining private destructor causes compilation error.

```c++
class Circle {
private:
  double radius;
  ~Circle();
public:
  Circle(double r) : radius(r) { };
  virtual double area(void);
  virtual double perimeter(void);
};
```

not compiled because of following error

```
error: use of undeclared identifier 'SWIGTYPE_Circle'
  SWIGTYPE_Circle->clientdata = &_Circle_clientData;
```