tools/resolve_btfids: Add /libsubcmd to .gitignore

Add libsubcmd to .gitignore, otherwise after compiling the kernel it
would result in the following:

    # bpf-next...bpf-next/master
    ?? tools/bpf/resolve_btfids/libsubcmd/

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/tencent_F13D670D5D7AA9C4BD868D3220921AAC090A@qq.com