Update service_extender stub generator to use 3.0's PlCoreBase class,
instead of the renamed XOSBase

Change-Id: I6a28f3ba3e18793a7c417a461bbffd8d759dbe9e
diff --git a/xos/genx/targets/service_extender.xtarget b/xos/genx/targets/service_extender.xtarget
index 5b4bc03..2488f2b 100644
--- a/xos/genx/targets/service_extender.xtarget
+++ b/xos/genx/targets/service_extender.xtarget
@@ -1,4 +1,4 @@
-from core.models.xosbase import *
+from core.models.plcorebase import *
 {% for m in proto.messages %}
 from models_decl import {{ m.name }}_decl
 {%- endfor %}