Fix KMail counting of unread messages in the system tray icon

Thanks to Francois Andriot
(cherry picked from commit 40c435e566)

kmail/kmsystemtray.cpp

@@ -493,7 +493,7 @@ void KMSystemTray::updateNewMessages()
    * our last known version, and adjust mCount with that difference */
   else
   {
-    int diff = unread - it.data();
+    int diff = unread - unread_it.data();
     mCount += diff;
   }