Cleanup warnings. (#5247)

From clang-tidy-9 and gcc-7: Invalid case style, narrowing definition, wrong
initialization order, unused variables.

include/xgboost/model.h

@@ -15,6 +15,7 @@ namespace xgboost {
 class Json;
 
 struct Model {
+  virtual ~Model() = default;
   /*!
    * \brief load the model from a json object
    * \param in json object where to load the model from
@@ -28,6 +29,7 @@ struct Model {
 };
 
 struct Configurable {
+  virtual ~Configurable() = default;
   /*!
    * \brief Load configuration from JSON object
    * \param in JSON object containing the configuration