lock SenseRepeatListLock when added a new sensor during script reconstitution.

This is already being done in the other place where a sensor is added.
Adding a sensor whilst another thread is iterating over the sensor list can cause a concurrency exception.

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/Plugins/SensorRepeat.cs

@@ -583,7 +583,9 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api.Plugins
                 ts.next =
                     DateTime.Now.ToUniversalTime().AddSeconds(ts.interval);
 
-                SenseRepeaters.Add(ts);
+                lock (SenseRepeatListLock)
+                    SenseRepeaters.Add(ts);
+                
                 idx += 6;
             }
         }