main: Fix unused variable warning on !HAVE_LIBSYSTEMD builds

This fixes:

      CC       main.o
    main.c: In function ‘listen_ports’:
    main.c:276:11: warning: unused variable ‘fds’ [-Wunused-variable]
      int ret, fds;
               ^

src/main.c

@@ -273,7 +273,10 @@ listen_ports(void *pool, struct cfg_st* config,
 {
 	struct addrinfo hints, *res;
 	char portname[6];
-	int ret, fds;
+	int ret;
+#ifdef HAVE_LIBSYSTEMD
+	int fds;
+#endif
 
 	list_head_init(&list->head);
 	list->total = 0;