Fix Windows build

Add reference to fix Windows build: no windows here to test, please report any issues back to IRC #opensim-dev ASAP

prebuild.xml

@@ -3264,6 +3264,7 @@
 
       <ReferencePath>../../../bin/</ReferencePath>
       <Reference name="System"/>
+      <Reference name="System.Core"/>
       <Reference name="System.Xml"/>
       <Reference name="System.Data"/>
       <Reference name="log4net" path="../../../bin/"/>