bpftool: Link zstd lib required by libelf

bpftool: Link zstd lib required by libelf

When the feature libelf-zstd is detected, the zstd lib is required by
libelf.  Link the zstd lib in this case.

Reviewed-by: Quentin Monnet <qmo@kernel.org>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Tested-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Guilherme Amadio <amadio@gentoo.org>
Cc: Hao Luo <haoluo@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Song Liu <song@kernel.org>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20241215221223.293205-4-leo.yan@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>