Fix propgrid sample compilation.

Resolve ambiguity in assignment of wxImage to wxBitmap in wxMSW which also has
wxBitmap::operator=(wxCursor). This should allow the sample to compile under
MSW after the changes of 1fce152d3c.

samples/propgrid/propgrid.cpp

@@ -1185,7 +1185,7 @@ static void RescaleBitmap(const wxBitmap& srcBmp, wxBitmap& dstBmp)
         // Use high-quality wxImage scaling functions
         wxImage img = srcBmp.ConvertToImage();
         img.Rescale(dstBmp.GetWidth(), dstBmp.GetHeight(), wxIMAGE_QUALITY_HIGH);
-        dstBmp = img;
+        dstBmp = wxBitmap(img);
 #else
         // Old method of scaling the image
         double scaleX = (double)dstBmp.GetWidth() / (double)srcBmp.GetWidth();