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>

tools/virtio/virtio_test.c

@@ -232,7 +232,7 @@ const struct option longopts[] = {
 	}
 };
 
-static void help()
+static void help(void)
 {
 	fprintf(stderr, "Usage: virtio_test [--help]"
 		" [--no-indirect]"