tools/bpf: sync btf.h header from kernel to tools

Sync include/uapi/linux/btf.h to tools/include/uapi/linux/btf.h.

Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>