ICMPv6: Send router advertisement from own link-local address

I'm not quite sure how I ended up doing this, but for some strange
reason the code before this commit is sending the ICMPv6 Router
Advertisements from some weird non-standard source address.  This is
a violation of RFC4861 which clearly states that the source address
of router advertisements "MUST be the link-local address assigned to the
interface from which this message is sent."

Change-Id: Ib444af70fc8f0b433d371281601fd5a37b29039e
diff --git a/ggsn/ggsn.h b/ggsn/ggsn.h
index 2dd963c..42f8e1c 100644
--- a/ggsn/ggsn.h
+++ b/ggsn/ggsn.h
@@ -78,6 +78,9 @@
 		struct osmo_fd fd;
 	} tun;
 
+	/* ipv6 link-local address */
+	struct in6_addr v6_lladdr;
+
 	struct apn_ctx_ip v4;
 	struct apn_ctx_ip v6;
 };