sched: track highest prio task queued

This patch adds accounting to each runqueue to keep track of the
highest prio task queued on the run queue. We only care about
RT tasks, so if the run queue does not contain any active RT tasks
its priority will be considered MAX_RT_PRIO.

This information will be used for later patches.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>