Add missing private constructor for static utility class LoginUrlToken

Change-Id: Ia9f81295de7542b6b090d378456fefc95cf6e891

gerrit-httpd/src/main/java/com/google/gerrit/httpd/LoginUrlToken.java

@@ -32,4 +32,7 @@ public class LoginUrlToken {
       return CharMatcher.is('/').trimLeadingFrom(Url.decode(encodedToken));
     }
   }
+
+  private LoginUrlToken() {
+  }
 }