tools/sched_ext/include: Add libbpf version guard for assoc_struct_ops

tools/sched_ext/include: Add libbpf version guard for assoc_struct_ops

Extract the inline bpf_program__assoc_struct_ops() call in SCX_OPS_LOAD()
into a __scx_ops_assoc_prog() helper and wrap it with a libbpf >= 1.7
version guard. bpf_program__assoc_struct_ops() was added in libbpf 1.7;
the guard provides a no-op fallback for older versions. Add the
<bpf/libbpf.h> include needed by the helper, and fix "assumming" typo in
a nearby comment.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Andrea Righi <arighi@nvidia.com>