BUILD: listener: silence a build warning about unused value without threads

A variable introduced in commit 1a43b9f32c ("MINOR: proto: extend
connection thread rebind API") is not used without threads and causes a
build warning. Let's just mark it maybe_unused.

Since the commit above is tagged for backporting, this one will need to
be backported along with it.

src/listener.c

@@ -1019,7 +1019,7 @@ static inline int listener_uses_maxconn(const struct listener *l)
  */
 void listener_accept(struct listener *l)
 {
-	void (*li_set_affinity2)(struct connection *);
+	void (*li_set_affinity2)(struct connection *) __maybe_unused;
 	struct connection *cli_conn;
 	struct proxy *p;
 	unsigned int max_accept;