route/link: add accessor API for IPv6 flags

Add functions to access the IPv6 specific flags of a link object.
Also the functions for IPv6 link flags translation are now exported, similar
to the non IPv6 specific translation functions.

https://github.com/thom311/libnl/pull/136
diff --git a/libnl-route-3.sym b/libnl-route-3.sym
index af1ed2d..2144185 100644
--- a/libnl-route-3.sym
+++ b/libnl-route-3.sym
@@ -1032,3 +1032,10 @@
 	rtnl_u32_set_selector;
 } libnl_3_2_28;
 
+libnl_3_4 {
+global:
+	rtnl_link_inet6_flags2str;
+	rtnl_link_inet6_str2flags;
+	rtnl_link_inet6_get_flags;
+	rtnl_link_inet6_set_flags;
+} libnl_3_2_29;