Add genl_socket_close() function

We've had genl_socket_open() but no corresponding genl_socket_close()
so far.  Let's fix that.
diff --git a/src/libgtpnl.map b/src/libgtpnl.map
index 9ce465d..804f8b3 100644
--- a/src/libgtpnl.map
+++ b/src/libgtpnl.map
@@ -1,6 +1,7 @@
 LIBGTPNL_1.0 {
 global:
   genl_socket_open;
+  genl_socket_close;
   genl_nlmsg_build_hdr;
   genl_socket_talk;
   genl_lookup_family;