IA64: Turn on CONFIG_HAVE_UNSTABLE_CLOCK

IA64: Turn on CONFIG_HAVE_UNSTABLE_CLOCK

commit 0773a6cf673316440999752e23f8c3d4f85e48b9 upstream.

sched_clock() on ia64 is based on ar.itc, so is never
completely synchronized between cpus. On some platforms
(e.g. certain models of SGI Altix) it may be running at
radically different frequencies.

Based on a patch from Dimitri Sivanich which set this
just for SN2 && GENERIC kernels ... it is needed for
all ia64 machines.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>