adjacency_matrix: iterator_concept

src/entt/graph/adjacency_matrix.hpp

@@ -30,6 +30,7 @@ public:
     using reference = value_type;
     using difference_type = std::ptrdiff_t;
     using iterator_category = std::input_iterator_tag;
+    using iterator_concept = std::forward_iterator_tag;
 
     constexpr edge_iterator() noexcept
         : it{},