[PATCH] Fix compilation of processor_idle.c on IA64

Broken earlier by me by a x86-64 patch.

The code was optimized away, but the compiler still complained about an
undeclared function.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>