lisa/_assets/kmodules/lisa: Include in-tree headers in sched_helpers.h

Fold including internal kernel headers required for the in-tree
build into sched_helpers.h. This way including sched_helpers.h will work
on its own in both workflows instead of requiring duplicated defines
wherever it is used.

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>