[cabinet] Add missing directory separator in FDICopy()

[Dan Kegel]

I just did a 'git pull' and rebuilt, and 'wine winemine' failed to
install gecko, complaining
err:appwizcpl:install_cab Could not extract package: 80070002

It seems that there are two places in fdi.c that the path and filename
are pasted together, and in one of them, the author forgot
to insert a directory separator.  Adding it fixed the problem.

(The code is eight years old, I guess Alexandre's recent changes
shook this bug loose.)
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