tools/sched_ext/include: Remove dead sdt_task_defs.h guard from common.h
The __has_include guard for sdt_task_defs.h is vestigial — the only
remaining content is the bpf_arena_common.h include which is available
unconditionally. Remove the dead guard.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Andrea Righi <arighi@nvidia.com>