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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/resolve_btfids/.gitignore

···
 /fixdep
 /resolve_btfids
 /libbpf/
+/libsubcmd/