Export c++ headers in CMake installation. (#4897)

* Move get transpose into cc.

* Clean up headers in host device vector, remove thrust dependency.

* Move span and host device vector into public.

* Install c++ headers.

* Short notes for c and c++.

Co-Authored-By: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

src/common/bitfield.h

@@ -13,7 +13,12 @@
 #include <string>
 #include <vector>
 
-#include "span.h"
+#if defined(__CUDACC__)
+#include <thrust/copy.h>
+#include <thrust/device_ptr.h>
+#endif  // defined(__CUDACC__)
+
+#include "xgboost/span.h"
 
 namespace xgboost {