Do `package require registry' only for windoze

OS: FreeBSD unix

After installing the indexCache patch I noticed that it
had no effect
with wish. It turns out that msgcat.tcl does a
`package require
registry' even if not running under windoze. This
causes an
unnecessary package scan every time wish starts for
non-windoze
systems since the registry package exists (I assume)
only for
windoze. I have attached one of many possible patches
to fix this.