BUILD: dns: avoid a build warning when threads are disabled (dss unused)

dns_session_release() only uses its struct dns_stream_server to access
the lock, so a warning is emitted when threads are disabled. Let's mark
it __maybe_unused.
diff --git a/src/dns.c b/src/dns.c
index cb30bff..69364eb 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -780,7 +780,7 @@
 static void dns_session_release(struct appctx *appctx)
 {
 	struct dns_session *ds = appctx->ctx.sft.ptr;
-	struct dns_stream_server *dss;
+	struct dns_stream_server *dss __maybe_unused;
 
 	if (!ds)
 		return;