IndexServletTest: Add missing serialVersionUID

Change-Id: I408a060abb7f28f5c9b98ef4dd304f44ed9da6b4
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-httpd/src/test/java/com/google/gerrit/httpd/raw/IndexServletTest.java

@@ -22,6 +22,8 @@ import org.junit.Test;
 
 public class IndexServletTest {
   class TestIndexServlet extends IndexServlet {
+    private static final long serialVersionUID = 1L;
+
     TestIndexServlet(String canonicalURL, String cdnPath) throws URISyntaxException {
       super(canonicalURL, cdnPath);
     }