core: make sched_runqueue public

Reverting b604832, because thread_yield() needs to access this
information.

core/include/sched.h

@@ -152,6 +152,11 @@ extern volatile int sched_num_threads;
  */
 extern volatile kernel_pid_t sched_active_pid;
 
+/**
+ * List of runqueues per priority level
+ */
+extern clist_node_t *sched_runqueues[SCHED_PRIO_LEVELS];
+
 /**
  * @brief   Lets current thread yield in favor of a higher prioritized thread.
  *

core/sched.c

@@ -45,7 +45,7 @@ volatile tcb_t *sched_active_thread;
 
 volatile kernel_pid_t sched_active_pid = KERNEL_PID_UNDEF;
 
-static clist_node_t *sched_runqueues[SCHED_PRIO_LEVELS];
+clist_node_t *sched_runqueues[SCHED_PRIO_LEVELS];
 static uint32_t runqueue_bitcache = 0;
 
 #if SCHEDSTATISTICS