libbpf: Add bpf_program__attach_kprobe_opts function

Adding bpf_program__attach_kprobe_opts that does the same
as bpf_program__attach_kprobe, but takes opts argument.

Currently opts struct holds just retprobe bool, but we will
add new field in following patch.

The function is not exported, so there's no need to add
size to the struct bpf_program_attach_kprobe_opts for now.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210714094400.396467-7-jolsa@kernel.org