tools/resolve_btfids: Use pkg-config to locate libelf

When libelf was not installed in the standard location, it cannot be
located by the current building config.

Use pkg-config to help locate libelf in such cases.

Signed-off-by: Shen Jiamin <shen_jiamin@comp.nus.edu.sg>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20221215044703.400139-1-shen_jiamin@comp.nus.edu.sg