notmuch setup: Fix to add maildir group (with comment) to existing file

The intent of "notmuch setup" is that it adds new, documented sections
to configuration files that were created before such sections were
defined. But to make this work, we have to explicitly set an option
in the maildir group if it didn't exist previously.

notmuch-config.c

@@ -344,7 +344,7 @@ notmuch_config_open (void *ctx,
 	g_key_file_get_boolean (config->key_file,
 				"maildir", "synchronize_flags", &error);
     if (error) {
-	config->maildir_synchronize_flags = TRUE;
+	notmuch_config_set_maildir_synchronize_flags (config, TRUE);
 	g_error_free (error);
     }