Fix compilation error CS0103 in UsingTask 'UnzipNuspec'

After adding Contrib.Nuget (as an dependency of SFML.Net) to my project, it failed to compile with error CS0103: "The name 'Thread' does not exist in current context."

I fixed the problem by 'using' namespace 'System.Threading' in UsingTask 'UnzipNuspec'.

My environment: Visual Studio Professional 2012, $(MSBuildToolsVersion) is '4.0'.