Added method for new listen_failed_alert port field

src/main/java/com/frostwire/jlibtorrent/alerts/ListenFailedAlert.java

@@ -51,6 +51,15 @@ public Operation getOperation() {
         return Operation.fromSwig(alert.getOperation());
     }
 
+    /**
+     * The port attempted to be opened for listening.
+     *
+     * @return
+     */
+    public int port() {
+        return alert.getPort();
+    }
+
     /**
      * The type of listen socket this alert refers to.
      *