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

since commit 108fc82596e3("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>

tools/virtio/.gitignore

@@ -2,3 +2,4 @@
 *.d
 virtio_test
 vringh_test
+virtio-trace/trace-agent