debug: Fix printing of new ids

The client side closure traces have incorrect object ids for new server
generated objects. This is because create_proxies() overwrites the id in
'n' type arguments by storing a pointer to the actual object in the 'o'
field of the union.

Getting back to an id from this pointer requires accessing a structure
that isn't visible outside of wayland-client.c.

Add a function pointer to fish the correct value out of the argument and
pass it to wl_closure_print.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>