ia64/pv_ops: avoid name conflict of get_irq_chip().

The macro get_irq_chip() is defined in linux/include/linux/irq.h
which cause name conflict with one in linux/arch/ia64/include/asm/paravirt.h.
rename the latter to __get_irq_chip().

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