bpftool: Fix handling enum64 in btf dump sorting

Wrong function is used to access the first enum64 element. Substituting btf_enum(t)
with btf_enum64(t) for BTF_KIND_ENUM64.

Fixes: 94133cf2 ("bpftool: Introduce btf c dump sorting")
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20240902171721.105253-1-mykyta.yatsenko5@gmail.com