stop_machine.h: fix implicit use of smp.h for smp_processor_id

This will show up on MIPS when we fix all the implicit header presences
that are because of module.h being everywhere.

In file included from kernel/trace/ftrace.c:16:
include/linux/stop_machine.h: In function 'stop_one_cpu':
include/linux/stop_machine.h:50: error: implicit declaration of function 'smp_processor_id'
include/linux/stop_machine.h: In function 'stop_cpus':
include/linux/stop_machine.h:80: error: implicit declaration of function 'raw_smp_processor_id'

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>