Fix ClassDB iterators making unintended copies

[BenLubar]

godot-cpp/src/core/class_db.cpp: In static member function 'static void godot::ClassDB::_editor_get_classes_used_callback(GDExtensionTypePtr)':
godot-cpp/src/core/class_db.cpp:395:88: error: loop variable 'pair' of type 'const std::pair<godot::StringName, const GDExtensionInstanceBindingCallbacks*>&' binds to a temporary constructed from type 'std::pair<const godot::StringName, const GDExtensionInstanceBindingCallbacks*>' [-Werror=range-loop-construct]
  395 | ame, const GDExtensionInstanceBindingCallbacks *> &pair : instance_binding_callbacks) {
      |                                                    ^~~~

godot-cpp/src/core/class_db.cpp:395:88: note: use non-reference type 'const std::pair<godot::StringName, const GDExtensionInstanceBindingCallbacks*>' to make the copy explicit or 'const std::pair<const godot::StringName, const GDExtensionInstanceBindingCallbacks*>&' to prevent copying

[dsnopek]

Thanks!

This error started happening on our CI as well, and this PR fixes it :-)