bpf: btf: Update tools/include/uapi/linux/btf.h with BTF ID

This patch sync the tools/include/uapi/linux/btf.h with
the newly introduced BTF ID support.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>