libbpf: Deprecate forgotten btf__get_map_kv_tids()

btf__get_map_kv_tids() is in the same group of APIs as
btf_ext__reloc_func_info()/btf_ext__reloc_line_info() which were only
used by BCC. It was missed to be marked as deprecated in [0]. Fixing
that to complete [1].

  [0] https://patchwork.kernel.org/project/netdevbpf/patch/20220201014610.3522985-1-davemarchevsky@fb.com/
  [1] Closes: https://github.com/libbpf/libbpf/issues/277



Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220203225017.1795946-1-andrii@kernel.org