Add applicable Requires on libxcrypt
Back in June, Björn Esser proposed to add OpenBSD-compatible bcrypt
support to our implementation of crypt(3), and Zack Weinberg replied
that it might actually make more sense to _remove_ libcrypt from
glibc, freeing up libcrypt.so.1 and crypt.h to be provided by a
separate project that could move faster. (For instance, libxcrypt:
https://github.com/besser82/libxcrypt)
This commit is the glibc part of:
https://fedoraproject.org/wiki/Changes/Replace_glibc_libcrypt_with_libxcrypt
