coresight: Fix implicit inclusion of linux/sched.h

The patch "Coresight: Add an interface for supporting ETM3/4 Context ID
tracing" adds uses of find_task_by_vpid() and task_pid_nr() from
linux/sched.h but does not include that header causing build errors in
at least an ARM allmodconfig where it is not implicitly included. Add an
explicit include to fix that.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>