main/swclock-offset: update dependency tree cache

If the current version of package "swclock-offset" is installed after system time was set (year 2021) and device rebootet (year 1970) but before the system time is updated again (year 2021), OpenRC cache gets confused and thinks the old dependency tree is the new one. Therefore, the freshly installed shutdown service isn't triggered, no offset file written and no effect of the package at first boot after installation. It works from the second boot on – but that's not the expected behaviour.

To resolve the confusion of OpenRC, the dependency tree cache is updated [1] within the post-install script of the openrc subpackage.

There is no additional post-upgrade script in this commit (e.g. to update the dependency tree cache on package upgrade) because the issue only affects the installation of the package.

[1] https://manpages.debian.org/buster/openrc/rc-update.8.en.html