Fix compile error on big endian

DestFormat is not defined here but should be Format_RGBX8888

Change-Id: Ie33989a6fc31650434281a6b7b0f4806524268af
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Timo Jyrinki <timo.jyrinki@canonical.com>
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

src/gui/image/qimage_conversions.cpp

@@ -1103,7 +1103,7 @@ static bool mask_alpha_converter_rgbx_inplace(QImageData *data, Qt::ImageConvers
         }
         rgb_data += pad;
     }
-    data->format = DestFormat;
+    data->format = QImage::Format_RGBX8888;
     return true;
 #endif
 }