libostree: set directory mtimes to 0 on checkout

libostree: set directory mtimes to 0 on checkout

We already set all file mtimes to 0 so that they are constant
over all checkouts, and can be made constant with a known value from
the system where the ostree was created.

However, this was not happening for directories. Zero their mtimes too.

This is important for shipping a fontconfig cache in the ostree;
the fontconfig cache files embed a directory mtime.