build: fix warnings on Clang 13+

Part of #6681

src/lib/core/crash.c

@@ -253,6 +253,7 @@ crash_report_feedback_daemon(struct crash_info *cinfo)
 	char *head = p;
 
 	int total = 0;
+	(void)total;
 	int size = 0;
 
 #define snprintf_safe(...) SNPRINT(total, snprintf, p, size, __VA_ARGS__)

src/lib/core/sio.c

@@ -54,6 +54,7 @@ static int
 sio_socketname_to_buffer(int fd, char *buf, int size)
 {
 	int n = 0;
+	(void)n;
 	SNPRINT(n, snprintf, buf, size, "fd %d", fd);
 	if (fd < 0)
 		return 0;