libbpf: Mark bpf_object__open_xattr() deprecated

Mark bpf_object__open_xattr() as deprecated, use
bpf_object__open_file() instead.

  [0] Closes: https://github.com/libbpf/libbpf/issues/287

Signed-off-by: Christy Lee <christylee@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220125010917.679975-1-christylee@fb.com