kcov: add missing #include <linux/sched.h>

In __sanitizer_cov_trace_pc we use task_struct and fields within it, but
as we haven't included <linux/sched.h>, it is not guaranteed to be
defined.  While we usually happen to acquire the definition through a
transitive include, this is fragile (and hasn't been true in the past,
causing issues with backports).

Include <linux/sched.h> to avoid any fragility.

[mark.rutland@arm.com: rewrote changelog]
Link: http://lkml.kernel.org/r/1481007384-27529-1-git-send-email-wangkefeng.wang@huawei.com


Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: James Morse <james.morse@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>