Add Fl_Valuator destructor.

This is required by Swig to generate code for wrapping
FLTK for Python and potentially other languages. It has
no impact on the API or ABI.

FL/Fl_Valuator.H

@@ -69,6 +69,8 @@ protected:
 
 public:
 
+  /** Destructor is accessible despite protected constructor. */
+  ~Fl_Valuator() FL_OVERRIDE { }
     /**    Sets the minimum (a) and maximum (b) values for the valuator widget. */
     void bounds(double a, double b) {min=a; max=b;}
     /**    Gets the minimum value for the valuator.  */