libbpf: Fix compilation warning due to mismatched printf format

On ppc64le architecture __s64 is long int and requires %ld. Cast to
ssize_t and use %zd to avoid architecture-specific specifiers.

Fixes: 4172843e ("libbpf: Fix signedness bug in btf_dump_array_data()")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220209063909.1268319-1-andrii@kernel.org