cgroup-defs: fix swork_event declare issue

The struct swork_event defined in work-simple.h which doesn't included before
it was used in cgroup-defs.h file. That cause a compiler error:

include/linux/cgroup-defs.h:144:21: error: field 'destroy_swork' has incomplete type

add this head file and remove compiler issue.

Signed-off-by: Alex Shi <alex.shi@linaro.org>
1 file changed