update zram-generator to upstream version 1.1.2

This commit imports the latest upstream version for zram-generator. Apart from the obvious changes (update build dependencies and fix build issues), the following are noteworthy:

• control: update long description (Fixes: lintian: rust-boilerplate)
• postinst: replace systemctl call with deb-systemd-invoke (Fixes: lintian: maintainer-script-calls-systemctl)
• zram-generator.conf: update from upstream example, while retaining previous behavior

/cc @claystan