sysutils/xfce4-settings: Improve build

Use distribution provided include file, and avoid regenerating it
and patching consumer.

Works around upstream issue:

https://gitlab.xfce.org/xfce/xfce4-settings/-/issues/564

Reported by: Olivier Duchateau <duchateau.olivier@gmail.com>
Obtained from: https://codeberg.org/olivierd/freebsd-xfce4/src/branch/main/sysutils/xfce4-settings/files/patch-xfsettingsd_Makefile.in