netlink: add forgotten opt_netlink header

sys/netlink/netlink_domain.c

@@ -30,6 +30,7 @@
  * This file contains socket and protocol bindings for netlink.
  */
 
+#include "opt_netlink.h"
 #include <sys/param.h>
 #include <sys/kernel.h>
 #include <sys/malloc.h>