Show correct progress when resuming update download

If you restart qTox in the middle of downloading an update, it would continue to download the update correctly but the progress bar would go from 0 to 50% instead of resuming from 50 to 100%

src/net/autoupdate.cpp

@@ -375,6 +375,7 @@ bool AutoUpdater::downloadUpdate()
         {
             qDebug() << "Skipping already downloaded file   '" + fileMeta.installpath+ "'";
             fileFile.close();
+            progressValue = initialProgress + step;
             continue;
         }