Reorder includes. (#5749)

* Reorder includes.

* R.

src/tree/constraints.cu

@@ -1,15 +1,15 @@
 /*!
  * Copyright 2019 XGBoost contributors
  */
+#include <algorithm>
+#include <string>
+#include <set>
+
 #include <thrust/copy.h>
 #include <thrust/device_vector.h>
 #include <thrust/execution_policy.h>
 #include <thrust/iterator/counting_iterator.h>
 
-#include <algorithm>
-#include <string>
-#include <set>
-
 #include "xgboost/logging.h"
 #include "xgboost/span.h"
 #include "constraints.cuh"