Make target private final in DelegatingClassLoader

This should not be visible outside of the class, or modified
after creation.

Change-Id: I7024a22e311fb7cdaffb0a7a79f935aecb7d6fb8

gerrit-server/src/main/java/com/google/gerrit/server/plugins/DelegatingClassLoader.java

@@ -21,7 +21,7 @@ import java.net.URL;
 import java.util.Enumeration;
 
 public class DelegatingClassLoader extends ClassLoader {
-  public ClassLoader target;
+  private final ClassLoader target;
 
   public DelegatingClassLoader(ClassLoader parent, ClassLoader target) {
     super(parent);