tools/bpf: Sync kernel btf.h header

The kernel uapi btf.h is synced to the tools directory.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>