Fix compilation on OS X with GCC 7 (#2256)

* Fix compilation on OS X with GCC 7

Compilation failed with

In file included from src/tree/tree_updater.cc:6:0:
include/xgboost/tree_updater.h:75:46: error: 'function' is not a member of 'std'
                                         std::function<TreeUpdater* ()> > {

caused by a missing <functional> include.

* Fixed another occurence of that issue spotted by @ClimberPG

include/xgboost/tree_updater.h

@@ -9,6 +9,7 @@
 #define XGBOOST_TREE_UPDATER_H_
 
 #include <dmlc/registry.h>
+#include <functional>
 #include <vector>
 #include <utility>
 #include <string>
@@ -16,10 +17,6 @@
 #include "./data.h"
 #include "./tree_model.h"
 
-#ifdef _MSC_VER
-#include <functional>
-#endif
-
 namespace xgboost {
 /*!
  * \brief interface of tree update module, that performs update of a tree.