Fix cpplint. (#4157)

* Add comment after #endif.
* Add missing headers.

src/tree/split_evaluator.cc

@@ -10,6 +10,7 @@
 #include <unordered_set>
 #include <vector>
 #include <limits>
+#include <memory>
 #include <string>
 #include <sstream>
 #include <utility>

src/tree/split_evaluator.h

@@ -11,6 +11,7 @@
 #include <dmlc/registry.h>
 #include <xgboost/base.h>
 #include <functional>
+#include <memory>
 #include <string>
 #include <utility>
 #include <vector>

src/tree/tree_updater.cc

@@ -37,6 +37,6 @@ DMLC_REGISTRY_LINK_TAG(updater_sync);
 #ifdef XGBOOST_USE_CUDA
 DMLC_REGISTRY_LINK_TAG(updater_gpu);
 DMLC_REGISTRY_LINK_TAG(updater_gpu_hist);
-#endif
+#endif  // XGBOOST_USE_CUDA
 }  // namespace tree
 }  // namespace xgboost

src/tree/updater_colmaker.cc

@@ -597,7 +597,7 @@ class ColMaker: public TreeUpdater {
       const auto num_features = static_cast<bst_omp_uint>(feat_set.size());
       #if defined(_OPENMP)
       const int batch_size = std::max(static_cast<int>(num_features / this->nthread_ / 32), 1);
-      #endif
+      #endif  // defined(_OPENMP)
       int poption = param_.parallel_option;
       if (poption == 2) {
         poption = static_cast<int>(num_features) * 2 < this->nthread_ ? 1 : 0;

src/tree/updater_quantile_hist.h

@@ -16,6 +16,7 @@
 #include <string>
 #include <queue>
 #include <iomanip>
+#include <unordered_map>
 #include <utility>
 
 #include "./param.h"