Make xmasked_view copy constructor default

Author: serge-sans-paille

include/xtensor/xmasked_view.hpp

@@ -160,6 +160,8 @@ namespace xt
         template <class D, class M>
         xmasked_view(D&& data, M&& mask);
 
+        xmasked_view(xmasked_view const&) = default;
+
         size_type size() const noexcept;
         const inner_shape_type& shape() const noexcept;
         const inner_strides_type& strides() const noexcept;