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

src/data/sparse_batch_page.h

@@ -17,16 +17,13 @@
 #include <string>
 #include <utility>
 #include <memory>
+#include <functional>
 
 #if DMLC_ENABLE_STD_THREAD
 #include <dmlc/concurrency.h>
 #include <thread>
 #endif
 
-#ifdef _MSC_VER
-#include <functional>
-#endif
-
 namespace xgboost {
 namespace data {
 /*!