Disambiguate use of RegionFlags in NullRegionData

This was not necessary with the nant build but is for xbuild, since xbuild includes associated project DLLs
And so picks up OpenMetaverse.dll even though it isn't referenced by OpenSim.Data.Null.dll

OpenSim/Data/Null/NullRegionData.cs

@@ -33,6 +33,7 @@ using OpenSim.Framework;
 using OpenSim.Data;
 using System.Reflection;
 using log4net;
+using RegionFlags = OpenSim.Framework.RegionFlags;
 
 namespace OpenSim.Data.Null
 {