htl: Hide __pthread_attr's __schedparam type [BZ #23088]

The content of the structure is only used internally, so we can make
__pthread_attr_getschedparam and __pthread_attr_setschedparam convert
between the public sched_param type and an internal __sched_param.

This allows to avoid to spuriously expose the sched_param type.

This fixes BZ #23088.