kernel/sched.c: add missing forward declaration for 'double_rq_lock'

Impact: build fix on certain configs

Added 'double_rq_lock' forward declaration, allowing double_rq_lock
to be used in _double_lock_balance().
Signed-off-by: NSteven Noonan <steven@uplinklabs.net>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -125,6 +125,9 @@ DEFINE_TRACE(sched_switch);
 DEFINE_TRACE(sched_migrate_task);
 
 #ifdef CONFIG_SMP
+
+static void double_rq_lock(struct rq *rq1, struct rq *rq2);
+
 /*
  * Divide a load by a sched group cpu_power : (load / sg->__cpu_power)
  * Since cpu_power is a 'constant', we can use a reciprocal divide.