hurd: fix warning

hurd: fix warning

timer_ptr2id and timer_id2ptr are used to convert between
application-visible timer_t and struct timer_node *. timer_ptr2id was made
to use void * instead of timer_t in 49b650430eb5 ('Update.') for no reason.
It happens that on Linux timer_t is void *, so both that change and this
commit are no-ops there, but not on systems where timer_t is not void *.

Using timer_ptr2id for filling sival_ptr also does not make sense since that
actually is a void *.

	* sysdeps/pthread/posix-timer.h (timer_ptr2id): Cast to timer_t
	instead of void *.
	* sysdeps/pthread/timer_create.c (timer_create): Do not use
	timer_ptr2id to cast struct timer_node * to void *.