bpftool: Add struct_ops map name

This patch adds BPF_MAP_TYPE_STRUCT_OPS to "struct_ops" name mapping
so that "bpftool map show" can print the "struct_ops" map type
properly.

[root@arch-fb-vm1 bpf]# ~/devshare/fb-kernel/linux/tools/bpf/bpftool/bpftool map show id 8
8: struct_ops  name dctcp  flags 0x0
	key 4B  value 256B  max_entries 1  memlock 4096B
	btf_id 7

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200115230037.1102674-1-kafai@fb.com