Hide internal __sched_setparam function [BZ #18822]

Hide internal __sched_setparam function to allow direct access within
libc.so and libc.a without using GOT nor PLT.

__GI___sched_setparam is defined when sysdeps/unix/syscalls.list is
used to generate sched_setparam.  Otherwise libc_hidden_def is needed
explicitly.

	[BZ #18822]
	* include/sched.h (__sched_setparam): Add libc_hidden_proto.
	* posix/sched_setp.c (__sched_setparam): Add libc_hidden_def.