tools/virtio: Ignore virtio-trace/trace-agent

since commit 108fc825

("tools: Add guest trace agent as a user tool")
introduce virtio-trace/trace-agent, it should be ignored in the git tree.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Message-Id: <tencent_52B2BC2F47540A5FEB46E710BD0C8485B409@qq.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>