[scudo] Add Last entry to ReleaseToOS enum. (#188645)

This allows static asserts to be set in tracing code that might use the
ReleaseToOS values as indexes.

This would have caused a compile failure instead of a runtime crash when
I added the use of a new ReleaseToOS value.

GitOrigin-RevId: 6b3556a6ea2fda31be3a615119d1c8d89452ce36