Revert "Reorder includes. (#5749)" (#5771)

This reverts commit d3a0efbf16.

src/tree/updater_gpu_hist.cu

@@ -1,6 +1,9 @@
 /*!
  * Copyright 2017-2020 XGBoost contributors
  */
+#include <thrust/copy.h>
+#include <thrust/reduce.h>
+#include <xgboost/tree_updater.h>
 #include <algorithm>
 #include <cmath>
 #include <memory>
@@ -9,10 +12,6 @@
 #include <utility>
 #include <vector>
 
-#include <thrust/copy.h>
-#include <thrust/reduce.h>
-
-#include "xgboost/tree_updater.h"
 #include "xgboost/host_device_vector.h"
 #include "xgboost/parameter.h"
 #include "xgboost/span.h"