setup: mark unused parameter in config callback

This is logically a continuation of 783a86c (config: mark unused
callback parameters, 2022-08-19), but this case was introduced much
later in 4412a04 (init.templateDir: consider this config setting
protected, 2024-03-29).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

setup.c

@@ -1907,7 +1907,7 @@ struct template_dir_cb_data {
 };
 
 static int template_dir_cb(const char *key, const char *value,
-			   const struct config_context *ctx, void *d)
+			   const struct config_context *ctx UNUSED, void *d)
 {
 	struct template_dir_cb_data *data = d;