[LIBXGBOOST] pass demo running.

src/logging.cc

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+/*!
+ * Copyright 2015 by Contributors
+ * \file logging.cc
+ * \brief Implementation of loggers.
+ * \author Tianqi Chen
+ */
+#include <xgboost/logging.h>
+#include <iostream>
+#include "./common/sync.h"
+
+namespace xgboost {
+ConsoleLogger::~ConsoleLogger() {
+  std::cout << log_stream_.str() << std::endl;
+}
+
+TrackerLogger::~TrackerLogger() {
+  log_stream_ << '\n';
+  rabit::TrackerPrint(log_stream_.str());
+}
+}  // namespace xgboost