[IA64] pvops: preparation: remove extern in irq_ia64.c

remove extern declaration of handle_IPI() in irq_ia64.c.
Instead, declare it in asm-ia64/smp.h.
Later handle_IPI() will be referenced from another file.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>