libbpf: Add new sec_def "sk_skb/verdict"

The new sec_def specifies sk_skb program type with
BPF_SK_SKB_VERDICT attachment type. This way, libbpf
will set expected_attach_type properly for the program.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240319175412.2941149-1-yonghong.song@linux.dev