[PATCH] Reinstate const in next_thread()

Before commit 47e65328, next_thread() took
a const task_t.  Reinstate the const qualifier, getting the next thread
never changes the current thread.

Signed-off-by: Keith Owens <kaos@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>