MIPS: Sibyte: Add missing sched.h header

MIPS: Sibyte: Add missing sched.h header

It's needed for the TASK_INTERRUPTIBLE definition.

Fixes the following build problem:
arch/mips/sibyte/common/sb_tbprof.c:235:4: error: 'TASK_INTERRUPTIBLE'
undeclared (first use in this function)

[ralf@linux-mips.org: Ideally sched.h should be included into the actual
user of TASK_INTERRUPTIBLE, <linux/wait.h> but that seems way too risky
that close to a release.]

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: sibyte-users@bitmover.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5479/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>