Make wxDataViewCtrl::IsMultiColumnSortAllowed() exist in all ports

Contrary to the documentation, this method only existed in the generic
control, add it to the base class now so that the code using it could compile
when using the native ports too.

include/wx/dataview.h

@@ -662,6 +662,9 @@ public:
         return !allow;
     }
 
+    // Return true if multi column sort is currently allowed.
+    virtual bool IsMultiColumnSortAllowed() const { return false; }
+
     // This should also be overridden to actually use the specified column for
     // sorting if using multiple columns is supported.
     virtual void ToggleSortByColumn(int WXUNUSED(column)) { }