t-reftable-readwrite: mark unused parameter in callback function

This spot was originally marked in in 4695c3f3a9 (reftable: mark unused
parameters in virtual functions, 2024-08-17), but was copied in
5b539a5361 (t: move reftable/readwrite_test.c to the unit testing
framework, 2024-08-13).

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

t/unit-tests/t-reftable-readwrite.c

@@ -26,7 +26,7 @@ static ssize_t strbuf_add_void(void *b, const void *data, size_t sz)
 	return sz;
 }
 
-static int noop_flush(void *arg)
+static int noop_flush(void *arg UNUSED)
 {
 	return 0;
 }