daemon: Work around old libnl3 failing to include linux/netlink.h

In file included from /usr/include/libnl3/netlink/socket.h:16:0,
                 from netdev.c:19:
/usr/include/libnl3/netlink/handlers.h:51:15: warning: ‘struct nlmsgerr’
declared inside parameter list will not be visible outside of this
definition or declaration
        struct nlmsgerr *nlerr, void *arg);

Change-Id: Ia2d1b0d5ccf427895feb80298d9b5c3aef1c6c9d
diff --git a/daemon/netdev.c b/daemon/netdev.c
index 2022d24..e619883 100644
--- a/daemon/netdev.c
+++ b/daemon/netdev.c
@@ -16,6 +16,7 @@
 #include <linux/if_tun.h>
 #include <sys/ioctl.h>
 
+#include <linux/netlink.h>
 #include <netlink/socket.h>
 #include <netlink/route/link.h>
 #include <netlink/route/addr.h>