tools/virtio: Add .gitignore for ringtest

Ignore executables for ringtest.
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Message-Id: <tencent_C121802C93CB4095C6D7D95113442E830A07@qq.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/ringtest/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
+/noring
+/ptr_ring
+/ring
+/virtio_ring_0_9
+/virtio_ring_inorder
+/virtio_ring_poll