x11-clocks/xclock: update from 1.1.1 to 1.2.0

Make portlint(1) happy by moving pkg-plist's records to the
Makefile.

ChangeLog: https://gitlab.freedesktop.org/xorg/app/xclock/-/compare/xclock-1.1.1...xclock-1.2.0?from_project_id=491

PR: 296352

Sponsored by: tipi.work