ia64/xen: add a necessary header file to compile include/xen/interface/xen.h

Create include/asm-ia64/pvclock-abi.h to compile which contains
the same definitions of include/asm-x86/pvclock-abi.h because ia64/xen
uses same structure.
Hopefully include/asm-x86/pvclock-abi.h would be moved to somewhere
more generic.

Another approach is to include include/asm-x86/pvclock-abi.h
from include/asm-ia64/pvclock-abi.h. But this would break
if/when x86 header files are moved under arch/x86.
So for now, same definitions are duplicated as suggested by Tony.

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