bpf: Fix bpftool without skeleton code enabled

Fix segfault from bpftool by adding emit_obj_refs_plain when skeleton
code is disabled.

Tested by deleting BUILD_BPF_SKELS in Makefile. We found this doing
backports for Cilium when a testing image pulled in latest bpf-next
bpftool, but kept using an older clang-7.

  # ./bpftool prog show
  Error: bpftool built without PID iterator support
  3: cgroup_skb  tag 7be49e3934a125ba  gpl
          loaded_at 2020-07-01T08:01:29-0700  uid 0
  Segmentation fault

Fixes: d53dee3fe0 ("tools/bpftool: Show info for processes holding BPF map/prog/link/btf FDs")
Reported-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/159375071997.14984.17404504293832961401.stgit@john-XPS-13-9370

tools/bpf/bpftool/pids.c

@@ -19,6 +19,7 @@ int build_obj_refs_table(struct obj_refs_table *table, enum bpf_obj_type type)
 	return -ENOTSUP;
 }
 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) {}
 
 #else /* BPFTOOL_WITHOUT_SKELETONS */