snapshot: try to suppress a warning from older versions of gcc

src/entt/entity/snapshot.hpp

@@ -172,8 +172,8 @@ public:
     basic_snapshot_loader(registry_type &source) noexcept
         : reg{&source} {
         // restoring a snapshot as a whole requires a clean registry
-        for([[maybe_unused]] auto [id, elem]: source.storage()) {
-            ENTT_ASSERT(elem.empty(), "Registry must be empty");
+        for([[maybe_unused]] auto elem: source.storage()) {
+            ENTT_ASSERT(elem.second.empty(), "Registry must be empty");
         }
     }