tools/resolve_btfids: Add /libbpf to .gitignore

This is what I see after compiling the kernel:

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

Fixes: fc6b48f6 ("tools/resolve_btfids: Build libbpf and libsubcmd in separate directories")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210212010053.668700-1-sdf@google.com