Add path_view difference_type

include/boost/url/impl/path_view.hpp

@@ -40,6 +40,8 @@ public:
         path_view::value_type;
     using pointer = value_type const*;
     using reference = value_type const&;
+    using difference_type =
+        std::ptrdiff_t;
     using iterator_category =
         std::forward_iterator_tag;