[GPU-Plugin] Fix gpu_hist to allow matrices with more than just 2^{32} elements. Also fixed CPU hist algorithm. (#2518)

tests/cpp/common/test_compressed_iterator.cc

@@ -1,5 +1,6 @@
 #include "../../../src/common/compressed_iterator.h"
 #include "gtest/gtest.h"
+#include <algorithm>
 
 namespace xgboost {
 namespace common {
@@ -51,4 +52,4 @@ TEST(CompressedIterator, Test) {
 }
 
 }  // namespace common
-}  // namespace xgboost
+}  // namespace xgboost