Make field final. #1555

Fixes `CanBeFinal` inspection violation.

Description:
>This inspection reports all fields, methods or classes, found in the specified inspection scope, that may have a final modifier added to their declarations.

src/main/java/com/puppycrawl/tools/checkstyle/gui/JTreeTable.java

@@ -105,7 +105,7 @@ public class JTreeTable extends JTable {
     /** For Serialisation that will never happen. */
     private static final long serialVersionUID = -8493693409423365387L;
     /** A subclass of JTree. */
-    protected TreeTableCellRenderer tree;
+    protected final TreeTableCellRenderer tree;
     private JTextArea editor;
     private List<Integer> lines2position;