Fix CPP lint. (#8807)

include/xgboost/metric.h

@@ -8,15 +8,16 @@
 #define XGBOOST_METRIC_H_
 
 #include <dmlc/registry.h>
-#include <xgboost/model.h>
-#include <xgboost/data.h>
 #include <xgboost/base.h>
+#include <xgboost/data.h>
 #include <xgboost/host_device_vector.h>
+#include <xgboost/model.h>
 
-#include <vector>
-#include <string>
 #include <functional>
+#include <memory>  // shared_ptr
+#include <string>
 #include <utility>
+#include <vector>
 
 namespace xgboost {
 struct Context;