Disable libxcrypt in PAM

It no longer builds: http://hydra.nixos.org/build/3046895

AFAIK we need libxcrypt for the legacy blowfish support, but it might
be better to apply this patch to Glibc:

http://cvs.pld-linux.org/cgi-bin/viewvc.cgi/cvs/packages/glibc/glibc-crypt-blowfish.patch?view=log