config_file: fix cast of readonly backend

In `backend_readonly_free`, the passed in config backend is being cast
to a `diskfile_backend` instead of to a `diskfile_readonly_backend`.
While this works out just fine because we only access its header values,
which were shared between both backends, it is undefined behaviour.

Use the correct type to fix this.

src/config_file.c

@@ -625,7 +625,7 @@ static int config_unlock_readonly(git_config_backend *_cfg, int success)
 
 static void backend_readonly_free(git_config_backend *_backend)
 {
-	diskfile_backend *backend = GIT_CONTAINER_OF(_backend, diskfile_backend, parent);
+	diskfile_readonly_backend *backend = GIT_CONTAINER_OF(_backend, diskfile_readonly_backend, parent);
 
 	if (backend == NULL)
 		return;