Reorder includes. (#5749)

* Reorder includes.

* R.

src/tree/gpu_hist/gradient_based_sampler.cu

@@ -1,15 +1,16 @@
 /*!
  * Copyright 2019 by XGBoost Contributors
  */
+#include <algorithm>
+#include <limits>
+
 #include <thrust/functional.h>
 #include <thrust/random.h>
 #include <thrust/transform.h>
+
 #include <xgboost/host_device_vector.h>
 #include <xgboost/logging.h>
 
-#include <algorithm>
-#include <limits>
-
 #include "../../common/compressed_iterator.h"
 #include "../../common/random.h"
 #include "gradient_based_sampler.cuh"

src/tree/gpu_hist/histogram.cu

@@ -1,12 +1,13 @@
 /*!
  * Copyright 2020 by XGBoost Contributors
  */
-#include <thrust/reduce.h>
-#include <thrust/iterator/transform_iterator.h>
 #include <algorithm>
 #include <ctgmath>
 #include <limits>
 
+#include <thrust/reduce.h>
+#include <thrust/iterator/transform_iterator.h>
+
 #include "xgboost/base.h"
 #include "row_partitioner.cuh"
 

src/tree/gpu_hist/row_partitioner.cu

@@ -1,10 +1,12 @@
 /*!
  * Copyright 2017-2019 XGBoost contributors
  */
+#include <vector>
+
 #include <thrust/iterator/discard_iterator.h>
 #include <thrust/iterator/transform_output_iterator.h>
 #include <thrust/sequence.h>
-#include <vector>
+
 #include "../../common/device_helpers.cuh"
 #include "row_partitioner.cuh"