tools/bpftool: Add ringbuf map to a list of known map types

Add symbolic name "ringbuf" to map to BPF_MAP_TYPE_RINGBUF. Without this,
users will see "type 27" instead of "ringbuf" in `map show` output.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200615225355.366256-1-andriin@fb.com