tools/virtio: fix build after 4.2 changes

more stubs, mostly

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/tools/virtio/linux/export.h b/tools/virtio/linux/export.h
new file mode 100644
index 0000000..416875e
--- /dev/null
+++ b/tools/virtio/linux/export.h
@@ -0,0 +1,3 @@
+#define EXPORT_SYMBOL_GPL(sym) extern typeof(sym) sym
+#define EXPORT_SYMBOL(sym) extern typeof(sym) sym
+