Fix dragimag sample compilation with modern compiler under MSW

Don't "override" UpdateBackingFromWindow() method of the base class
unless it really exists, which is only the case for the generic
wxDragImage implementation but not the native MSW one.

See https://github.com/wxWidgets/wxWidgets/pull/1671

samples/dragimag/dragimag.cpp

@@ -488,6 +488,7 @@ bool DragShape::Draw(wxDC& dc, bool highlight)
         return false;
 }
 
+#if wxUSE_GENERIC_DRAGIMAGE
 // MyDragImage
 
 // On some platforms, notably Mac OS X with Core Graphics, we can't blit from
@@ -503,4 +504,4 @@ bool MyDragImage::UpdateBackingFromWindow(wxDC& WXUNUSED(windowDC), wxMemoryDC&
     m_canvas->DrawShapes(destDC);
     return true;
 }
-
+#endif