ipgre: add support for gretap tunnel

Since kernel support both gre/gretap tunnel, so add support
for gretap appropriately.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>

[thaller@redhat.com: modified original patch to move symbols
 in libnl-route-3.sym to proper section]

http://lists.infradead.org/pipermail/libnl/2016-April/002102.html
diff --git a/libnl-route-3.sym b/libnl-route-3.sym
index 1fd205b..cd2b824 100644
--- a/libnl-route-3.sym
+++ b/libnl-route-3.sym
@@ -893,6 +893,8 @@
 	rtnl_link_bridge_pvid;
 	rtnl_link_is_macvtap;
 	rtnl_link_is_vrf;
+	rtnl_link_ipgretap_add;
+	rtnl_link_ipgretap_alloc;
 	rtnl_link_macvtap_alloc;
 	rtnl_link_macvtap_flags2str;
 	rtnl_link_macvtap_get_flags;