Allow to use Destructible as virtual base class

This is in preparation for a pure-virtual base class for
PluginInsert to expose `DropReferences` in the virtual base.

```
class PlugInsertBase : virtual public PBD::Destructible
class PluginInsert : public Processor, public PlugInsertBase
class Processor : public SessionObject
class SessionObject : public PBD::StatefulDestructible
```

libs/pbd/pbd/destructible.h

@@ -25,8 +25,7 @@
 namespace PBD {
 
 class LIBPBD_API Destructible {
-  public:
-        Destructible() {}
+public:
 	virtual ~Destructible () { Destroyed(); }
 
 	PBD::Signal0<void> Destroyed;

libs/pbd/pbd/statefuldestructible.h

@@ -26,7 +26,7 @@
 namespace PBD {
 
 /** Base class for objects with saveable and undoable state with destruction notification */
-class LIBPBD_API StatefulDestructible : public Stateful, public Destructible
+class LIBPBD_API StatefulDestructible : public Stateful, virtual public Destructible
 {
 };