bpf: Fix build error in case of !CONFIG_DEBUG_INFO_BTF

BTF_ID_FLAGS macro needs to be able to take 0 or 1 args, so make it a
variable argument. BTF_SET8_END is incorrect, it should just be empty.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: ab21d606

 ("bpf: Introduce 8-byte BTF set")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220722113605.6513-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>