rcu: Fix mismatched variable in rcutree_trace.c

rcutree.c defines rcu_cpu_kthread_cpu as int, not unsigned int,
so the extern has to follow that.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>