[bpf-next PATCH v2] bpf: fix bpftool without skeleton code enabled

[John Fastabend <john.fastabend@xxxxxxxxx>]

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

Reported-by: Joe Stringer <joe@xxxxxxxxxxx>
Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
---
 tools/bpf/bpftool/pids.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/bpf/bpftool/pids.c b/tools/bpf/bpftool/pids.c
index 2709be4de2b1..7d5416667c85 100644
--- a/tools/bpf/bpftool/pids.c
+++ b/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 */