wlsunset: Start after graphical-session

[jervw]

Description

Fixes the wlsunset service to correctly start after graphical-session.target is initialized. Previously, wlsunset was unable to start on some Wayland compositors like Niri because the display was not yet initialized at the time of service activation.

Checklist

• Change is backwards compatible.

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Maintainer CC

@matrss