gtp-rtnl: add gtp_dev_config function

This function allows us to set the gtp0 device configuration the route
to encapsulate all traffic that is addressed to the MS from the GGSN
diff --git a/src/libgtpnl.map b/src/libgtpnl.map
index 2e80543..6e69ef8 100644
--- a/src/libgtpnl.map
+++ b/src/libgtpnl.map
@@ -6,6 +6,7 @@
   genl_lookup_family;
 
   gtp_dev_create;
+  gtp_dev_config;
   gtp_dev_destroy;
 
   gtp_add_tunnel;