Allow external log4j.properties to use HttpLogLayout

When using an external custom log4j.properties using the 
-Dlog4j.configuration option, you may need to keep the 
ability to log HTTP activity using the Gerrit HttpLogLayout.

Change-Id: Ibd4b36736fa0f716c346d41f5586f8e66a5d1431

gerrit-pgm/src/main/java/com/google/gerrit/pgm/http/jetty/HttpLogLayout.java

@@ -26,7 +26,7 @@ public final class HttpLogLayout extends Layout {
   private long lastTimeMillis;
   private String lastTimeString;
 
-  HttpLogLayout() {
+  public HttpLogLayout() {
     final TimeZone tz = TimeZone.getDefault();
     dateFormat = new SimpleDateFormat("dd/MMM/yyyy:HH:mm:ss Z");
     dateFormat.setTimeZone(tz);