girwriter: Fix writing type for out params

The extra '*' is required for g_type_info_is_pointer() to
return TRUE. This fixes PyGObject marshaling for an interface
vfunc with a Boxed out param.

https://bugzilla.gnome.org/show_bug.cgi?id=741604
Signed-off-by: Garrett Regier <garrett.regier@riftio.com>