sparc64: fix sparse warning in process_64.c

Fix following warning:
process_64.c:91:25: warning: non-ANSI function declaration of function 'arch_cpu_idle_dead'

Add proper (void) to function definition

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>