munmap the tzdata file if ZoneInfoDB.TzData is ever collected

In the unusual / unexpected case that the runtime is shut down
cleanly but the process remains we should be munmapping the
MappedFile explicitly. MappedFile does not have a finalizer
that unmaps automatically.

Change-Id: I51c364cbe0f356798dbb73755277da34d97bf452
1 file changed