Changes

contrib/generate_build_dep.sh: pass '-f' to mv

It may happen that the same directory is present in both
`_build/default/lib` and `_build/default/plugins` directories.
In this case the `mv` command will fail, and this is exactly
what happens when adding a new dependency in a follow-up patch:

mv: cannot move './_build/default/plugins/ncalendar' to '_checkouts/ncalendar': Directory not empty
mv: cannot move './_build/default/plugins/ndto' to '_checkouts/ndto': Directory not empty
mv: cannot move './_build/default/plugins/njson' to '_checkouts/njson': Directory not empty

Pass `-f` to allow overwriting already existing checkouts.

Change-Id: I9a95afcac54dec9684f34224a912574bb387e96c