libxcrypt: fix hard-coded ".so" extension

2 issues:
- the .so extension is hard-coded, and therefore the libxcryt package compiled with
  meta-darwin is empty, because the dylib files are not contained in FILES_${PN}
- nothing actually produces a file libcrypt-*.so (the symlink file is libcrypt.so, without dash), thus
  defining FILES:${PN} manually to contain libcrypt-*.so has no effect.

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 7ed6bfa2)
Signed-off-by: Steve Sakoman <steve@sakoman.com>