tools:virtio: fix compilation warning

We do not allow old-style function definition.  Always spell foo(void) if
a function does not take any parameters.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c
index e626fa5..6d25dcd 100644
--- a/tools/virtio/virtio_test.c
+++ b/tools/virtio/virtio_test.c
@@ -232,7 +232,7 @@
 	}
 };
 
-static void help()
+static void help(void)
 {
 	fprintf(stderr, "Usage: virtio_test [--help]"
 		" [--no-indirect]"