shell: Add the right output object to the shell output_list

Error on my part while renaming the shell_output variable.
diff --git a/src/shell.c b/src/shell.c
index 8f17e89..fdc163e 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -4651,7 +4651,7 @@
 	shell_output->destroy_listener.notify = handle_output_destroy;
 	wl_signal_add(&output->destroy_signal,
 		      &shell_output->destroy_listener);
-	wl_list_insert(shell->output_list.prev, &output->link);
+	wl_list_insert(shell->output_list.prev, &shell_output->link);
 }
 
 static void