Fix build break from 1aa0271 by adding System.Core to prebuild.xml
Fix build break from 1aa0271 by adding System.Core to prebuild.xml

justinccdev/opensim,ft-/arribasim-dev-tests,rryk/omp-server,bravelittlescientist/opensim-performance,bravelittlescientist/opensim-performance,RavenB/opensim,M-O-S-E-S/opensim,OpenSimian/opensimulator,ft-/opensim-optimizations-wip,TomDataworks/opensim,EriHoss/OpenSim_0.8.2.0_Dev_LibLSLCC,bravelittlescientist/opensim-performance,RavenB/opensim,TomDataworks/opensim,ft-/arribasim-dev-extras,ft-/opensim-optimizations-wip-extras,BogusCurry/arribasim-dev,ft-/opensim-optimizations-wip-tests,ft-/opensim-optimizations-wip-tests,bravelittlescientist/opensim-performance,QuillLittlefeather/opensim-1,ft-/opensim-optimizations-wip,EriHoss/OpenSim_0.8.2.0_Dev_LibLSLCC,ft-/arribasim-dev-tests,TomDataworks/opensim,TomDataworks/opensim,QuillLittlefeather/opensim-1,ft-/arribasim-dev-extras,rryk/omp-server,BogusCurry/arribasim-dev,RavenB/opensim,justinccdev/opensim,ft-/opensim-optimizations-wip,ft-/arribasim-dev-extras,BogusCurry/arribasim-dev,ft-/arribasim-dev-tests,ft-/arribasim-dev-tests,OpenSimian/opensimulator,TomDataworks/opensim,ft-/opensim-optimizations-wip-tests,EriHoss/OpenSim_0.8.2.0_Dev_LibLSLCC,OpenSimian/opensimulator,M-O-S-E-S/opensim,ft-/arribasim-dev-extras,Michelle-Argus/ArribasimExtract,M-O-S-E-S/opensim,QuillLittlefeather/opensim-1,Michelle-Argus/ArribasimExtract,ft-/arribasim-dev-tests,OpenSimian/opensimulator,RavenB/opensim,ft-/opensim-optimizations-wip-tests,BogusCurry/arribasim-dev,EriHoss/OpenSim_0.8.2.0_Dev_LibLSLCC,EriHoss/OpenSim_0.8.2.0_Dev_LibLSLCC,Michelle-Argus/ArribasimExtract,Michelle-Argus/ArribasimExtract,ft-/opensim-optimizations-wip-tests,ft-/opensim-optimizations-wip-extras,rryk/omp-server,OpenSimian/opensimulator,ft-/arribasim-dev-extras,rryk/omp-server,BogusCurry/arribasim-dev,QuillLittlefeather/opensim-1,QuillLittlefeather/opensim-1,bravelittlescientist/opensim-performance,ft-/arribasim-dev-extras,ft-/opensim-optimizations-wip-extras,justinccdev/opensim,M-O-S-E-S/opensim,TomDataworks/opensim,rryk/omp-server,justinccdev/opensim,OpenSimian/opensimulator,ft-/opensim-optimizations-wip-extras,RavenB/opensim,bravelittlescientist/opensim-performance,ft-/opensim-optimizations-wip,M-O-S-E-S/opensim,RavenB/opensim,RavenB/opensim,EriHoss/OpenSim_0.8.2.0_Dev_LibLSLCC,justinccdev/opensim,QuillLittlefeather/opensim-1,ft-/opensim-optimizations-wip-extras,justinccdev/opensim,OpenSimian/opensimulator,EriHoss/OpenSim_0.8.2.0_Dev_LibLSLCC,M-O-S-E-S/opensim,M-O-S-E-S/opensim,Michelle-Argus/ArribasimExtract,BogusCurry/arribasim-dev,rryk/omp-server,QuillLittlefeather/opensim-1,Michelle-Argus/ArribasimExtract,ft-/arribasim-dev-tests,TomDataworks/opensim