CObj: default args in constructors result in compile time error

Default args in constructor of CObj classes are currently not possible:
```
@CObj
class GtkWindow(windowType: GtkWindowType = GtkWindow.TOPLEVEL)
```
results in 
```
scala.scalanative.util.UnsupportedException: methods in extern objects must have extern body
```

SN 0.3.6, obj-interop 0.0.2-SNAPSHOT