gtp-rtnl: add gtp_dev_destroy

This function allows you to destroy the gtpX device.
diff --git a/src/libgtpnl.map b/src/libgtpnl.map
index b38785c..326bf17 100644
--- a/src/libgtpnl.map
+++ b/src/libgtpnl.map
@@ -4,7 +4,10 @@
   genl_nlmsg_build_hdr;
   genl_socket_talk;
   genl_lookup_family;
+
   gtp_dev_create;
+  gtp_dev_destroy;
+
   gtp_add_tunnel;
   gtp_del_tunnel;
   gtp_list_tunnel;