tools/bpf: bpftool, add xskmap in map types

When listed all maps, bpftool currently shows (null) for xskmap.
Added xskmap type in map_type_name[] to show correct type.

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>