host-mkpasswd: fix crash on Fedora 28 build host

host-mkpasswd: fix crash on Fedora 28 build host

crypt() is an optional glibc feature. Some distros, like Fedora 28, are
phasing it out to be replaced with libxcrypt [1]. Unfortunately this
change is only ABI compatible, not source code compatible, i.e. the code
will compile with warnings about undefined crypt(), but the resulting
binary will crash.

Follow the guidance in the Fedora bug and include crypt.h when
_XOPEN_CRYPT is not defined.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1566464

Signed-off-by: Stefan Becker <chemobejk@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>