[view] Move view types out of the experimental namespace

Author: ldionne

include/boost/hana/fwd/view.hpp

@@ -13,6 +13,8 @@ Distributed under the Boost Software License, Version 1.0.
 #include <boost/hana/config.hpp>
 #include <boost/hana/fwd/core/make.hpp>
 
+#include <cstddef>
+
 
 BOOST_HANA_NAMESPACE_BEGIN
     //! @ingroup group-datatypes