Forgot to add final

Author: harawata

src/main/java/org/apache/ibatis/mapping/ParameterMapping.java

@@ -27,7 +27,7 @@
  */
 public class ParameterMapping {
 
-  private static Object UNSET = new Object();
+  private static final Object UNSET = new Object();
   private Configuration configuration;
 
   private String property;