Add line accidentally left out of recent non-root prim sit fix

Original commit is ff4e7de7

OpenSim/Region/Framework/Scenes/ScenePresence.cs

@@ -2314,6 +2314,8 @@ namespace OpenSim.Region.Framework.Scenes
 
                 // An viewer expects to specify sit positions as offsets to the root prim, even if a child prim is
                 // being sat upon.
+                offset += part.OffsetPosition;
+
                 ControllingClient.SendSitResponse(
                     part.ParentGroup.UUID, offset, sitOrientation, false, cameraAtOffset, cameraEyeOffset, forceMouselook);