x11/xfce4-screensaver: Fix build when PAM option is off

When PAM option is off a different file is compiled, which, since
version 4.18.3, tries to include crypt.h. Such file is not present in
FreeBSD. The relevant definitions are in unistd.h.

Fix by adding ifdef to skip such problematic include.

Reported upstream: https://gitlab.xfce.org/apps/xfce4-screensaver/-/merge_requests/29

PR: 277763