[IA64] export account_system_vtime

The symbol account_system_vtime is used by the kvm module but
not exported.  This breaks building with CONFIG_VIRT_CPU_ACCOUNTING
and CONFIG_KVM=m.

Signed-off-by: Doug Chapman <doug.chapman@hp.com>
Acked-by: Hidetosho Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>