sysutils/logrotate: Fix Logrotate to work by default

If the file /var/log/lastlog is missing, logrotate fails, even if other
configs are available.

Add "missingok" to this block so other logrorate blocks located in
${PREFIX}/etc/logrotate.d/ will function as expected when that one file
is unavailable.

While here remove unnecessary GNU_CONFIGURE_MANPREFIX.

PR: 293024
Pull Request: https://github.com/freebsd/freebsd-ports/pull/472/