selftests/bpf: Add bpf_get_socket_cookie to bpf_helpers.h

Add missing helper to bpf_helpers.h that is used in tests and samples.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>