Fix import of GetFileHash & VerifyFileHash tasks (#3999)
Fix import of GetFileHash & VerifyFileHash tasks (#3999)

The implementation of these tasks already existed.  However, these tasks
are quite new, and their existing `<UsingTask>` element referenced the
wrong assembly.
mono/msbuild,mono/msbuild,Microsoft/msbuild,Microsoft/msbuild,chipitsine/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,rainersigwald/msbuild,Microsoft/msbuild,cdmihai/msbuild,chipitsine/msbuild,jeffkl/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,mono/msbuild,cdmihai/msbuild,Microsoft/msbuild,cdmihai/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,rainersigwald/msbuild,mono/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild