[IA64] Correct definition of handle_IPI

The declaration of handle_IPI in arch/ia64/kernel/smp.c was changed but
not the definition of this function.  Remove struct pt_regs from
handle_IPI().

Signed-off-by: Keith Owens <kaos@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>