bpf: Fix another bpftool segfault without skeleton code enabled

emit_obj_refs_json needs to added the same as with emit_obj_refs_plain
to prevent segfaults, similar to Commit "8ae4121b bpf: Fix bpftool
without skeleton code enabled"). See the error below:

    # ./bpftool -p prog
    {
        "error": "bpftool built without PID iterator support"
    },[{
            "id": 2,
            "type": "cgroup_skb",
            "tag": "7be49e3934a125ba",
            "gpl_compatible": true,
            "loaded_at": 1594052789,
            "uid": 0,
            "bytes_xlated": 296,
            "jited": true,
            "bytes_jited": 203,
            "bytes_memlock": 4096,
            "map_ids": [2,3
    Segmentation fault (core dumped)

The same happens for ./bpftool -p map, as well as ./bpftool -j prog/map.

Fixes: d53dee3f ("tools/bpftool: Show info for processes holding BPF map/prog/link/btf FDs")
Signed-off-by: Louis Peens <louis.peens@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200708110827.7673-1-louis.peens@netronome.com

tools/bpf/bpftool/pids.c

@@ -20,6 +20,7 @@ int build_obj_refs_table(struct obj_refs_table *table, enum bpf_obj_type type)
 }
 void delete_obj_refs_table(struct obj_refs_table *table) {}
 void emit_obj_refs_plain(struct obj_refs_table *table, __u32 id, const char *prefix) {}
+void emit_obj_refs_json(struct obj_refs_table *table, __u32 id, json_writer_t *json_writer) {}
 
 #else /* BPFTOOL_WITHOUT_SKELETONS */