Suppress -Wunused-result warning about write().

pg_test_fsync's signal_cleanup() intentionally ignores the write()
result since there's not much we could do about it, but certain
compilers make that harder than it ought to be.

This was missed in commit 52e98d4502.

Reviewed-by: Tristan Partin, Peter Eisentraut
Discussion: https://postgr.es/m/20231206161839.GA2828158%40nathanxps13

src/bin/pg_test_fsync/pg_test_fsync.c

@@ -598,11 +598,14 @@ test_non_sync(void)
 static void
 signal_cleanup(SIGNAL_ARGS)
 {
+	int			rc;
+
 	/* Delete the file if it exists. Ignore errors */
 	if (needs_unlink)
 		unlink(filename);
 	/* Finish incomplete line on stdout */
-	write(STDOUT_FILENO, "\n", 1);
+	rc = write(STDOUT_FILENO, "\n", 1);
+	(void) rc;					/* silence compiler warnings */
 	_exit(1);
 }