Fix: Include linux/sched/rt.h for kernels v3.9 to v3.14

Fix: Include linux/sched/rt.h for kernels v3.9 to v3.14

From kernel v3.0 to v3.8, MAX_RT_PRIO is defined in linux/sched.h.

From kernel v3.9 to v3.14, MAX_RT_PRIO is defined in linux/sched/rt.h,
which is not included by linux/sched.h (hence this work-around).

From kernel v3.15 onwards, MAX_RT_PRIO is defined in linux/sched/prio.h,
which is included by linux/sched.h.

Add the missing linux/sched/rt.h include for the affected kernel version
range.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ie7e1d9dc710621deca04553a9b5ba7f9a4d83c15