BUG/MINOR: proxy: fix dyncookie_key leak on deinit()

proxy dyncookie_key wasn't cleaned up in free_proxy(), resulting in small
memory leak if "dynamic-cookie-key" was used on a regular or default
proxy.

It may be backported to all stable versions.

src/proxy.c

@@ -221,6 +221,7 @@ void free_proxy(struct proxy *p)
 	istfree(&p->server_id_hdr_name);
 	ha_free(&p->check_command);
 	ha_free(&p->check_path);
+	ha_free(&p->dyncookie_key);
 	free(p->rdp_cookie_name);
 	free_email_alert(p);
 	free(p->invalid_rep);