ia64: move function declaration, ia64_cpu_local_tick() from .c to .h

eliminate the function declaration ia64_cpu_local_tick() in
process.c by defining in arch/ia64/include/asm/timex.h
The same function will be used in a different .c file later.

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