Don't generate constructor wrappers if a base class has a private constructor

g++-5 errors out with this now with errors such as:

default_constructor_wrap.cxx:665:27: error: use of deleted function ‘FFF::FFF()’
   result = (FFF *)new FFF();
                           ^
default_constructor_wrap.cxx:314:7: note: ‘FFF::FFF()’ is implicitly deleted because the default definition would be ill-formed:
 class FFF : public F {
       ^
default_constructor_wrap.cxx:301:4: error: ‘F::~F()’ is private
    ~F() { }
    ^
default_constructor_wrap.cxx:314:7: error: within this context

Examples/test-suite/go/default_constructor_runme.go

@@ -21,9 +21,6 @@ func main() {
 	f := dc.NewF()
 	f.Destroy()
 
-	ff := dc.NewFFF()
-	ff.Destroy()
-
 	g := dc.NewG()
 
 	dc.GDestroy(g)