Suppress a Sparse warning

Move the export to a file used by both qdev.c and sysbus.c.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/qdev.h b/hw/qdev.h
index 11744fa..59ac8dc 100644
--- a/hw/qdev.h
+++ b/hw/qdev.h
@@ -162,4 +162,7 @@
 void qdev_prop_register_compat(CompatProperty *props);
 void qdev_prop_set_compat(DeviceState *dev);
 
+/* This is a nasty hack to allow passing a NULL bus to qdev_create.  */
+extern struct BusInfo system_bus_info;
+
 #endif