meson-build: Add polkit option

As it's made for mousepad. Unfortunately, it is not possible to ensure that the polkit.its file, required to run i18n.merge_file(), will be associated with polkit-gobject-1, or even with any pkg-config file. But this is the case on several distributions, so it's better than nothing, and allows users not to install the org.xfce.thunar.policy file if they don't need it and don't want to bother with this.

Related: !438 (comment 86765) Related: https://mail.xfce.org/pipermail/xfce4-dev/2025-March/033308.html