Move ellpack page construction into DMatrix (#4833)

src/data/ellpack_page.cc

@@ -0,0 +1,25 @@
+/*!
+ * Copyright 2019 XGBoost contributors
+ *
+ * \file ellpack_page.cc
+ */
+#ifndef XGBOOST_USE_CUDA
+
+#include <xgboost/data.h>
+
+// dummy implementation of ELlpackPage in case CUDA is not used
+namespace xgboost {
+
+class EllpackPageImpl {};
+
+EllpackPage::EllpackPage(DMatrix* dmat) {
+  LOG(FATAL) << "Internal Error: XGBoost is not compiled with CUDA but EllpackPage is required";
+}
+
+EllpackPage::~EllpackPage() {
+  LOG(FATAL) << "Internal Error: XGBoost is not compiled with CUDA but EllpackPage is required";
+}
+
+}  // namespace xgboost
+
+#endif  // XGBOOST_USE_CUDA