Avoid a compile error on hosts with libevent too old for EVENT_LOG_WARN.

This uses _EVENT_LOG_WARN instead, which appears to be defined in the
 old versions of libevent that I have on some systems.

src/httpserver.cpp

@@ -323,6 +323,10 @@ static void HTTPWorkQueueRun(WorkQueue<HTTPClosure>* queue)
 /** libevent event log callback */
 static void libevent_log_cb(int severity, const char *msg)
 {
+#ifndef EVENT_LOG_WARN
+// EVENT_LOG_WARN was added in 2.0.19; but before then _EVENT_LOG_WARN existed.
+# define EVENT_LOG_WARN _EVENT_LOG_WARN
+#endif
     if (severity >= EVENT_LOG_WARN) // Log warn messages and higher without debug category
         LogPrintf("libevent: %s\n", msg);
     else