Run OpenSourceSign target after Compile target

It used to run before 'CopyFilesToOutputDirectory', but that target has a different name on Mono's xbuild (namely 'DeployOutputFiles'), so it doesn't run there.

Using AfterTargets="Compile" shouldn't make a difference and works on Mono as well.

Note: I'm not that well versed in the intricacies of MSBuild targets, so if this causes problems on Windows please let me know.