BUG: Correctly encode archive file name

Use qPrintable() (which uses QString::toLocal8Bit() under the hood) to
encode the QString archive file name to char const* for passing to
extract_tar, rather than QString::toLatin1(), which is unlikely to
produce the correct bytes (the underlying system calls are more likely
to want system locale encoding) in case of a non-ASCII file name.

This sort of thing appears to be endemic to Slicer, but I don't care to
propagate that to the forthcoming call to list_archive(); thus, making
it consistent in this file at least.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23400 3bd1e089-480b-0410-8dfb-8563597acbee