libbpf: Name changing for btf_get_from_id

s/btf_get_from_id/btf__get_from_id/ to restore the API naming convention.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>