stop_machine: some compile fix

fix compile error:
kernel/stop_machine.c:139:21: error: static declaration of 'stop_cpus_mutex' follows non-static declaration
 static DEFINE_MUTEX(stop_cpus_mutex);
                     ^
In file included from kernel/stop_machine.c:18:0:
include/linux/stop_machine.h:30:21: note: previous declaration of 'stop_cpus_mutex' was here
 extern struct mutex stop_cpus_mutex;
                     ^