checkstyle.xml: add the FinalClass module

config/checkstyle/checkstyle.xml

@@ -414,6 +414,7 @@
         <!-- Checks for class design                  -->
         <!-- See https://checkstyle.org/checks/design -->
         <module name="DesignForExtension"/>
+        <module name="FinalClass"/>
         <module name="HideUtilityClassConstructor"/>
         <module name="InterfaceIsType"/>
     </module>