libbpf: Add bpf_object pointer to kernel_supports().

Add a pointer to 'struct bpf_object' to kernel_supports() helper.
It will be used in the next patch.
No functional changes.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210514003623.28033-13-alexei.starovoitov@gmail.com