zram-generator: fix build and improve packaging

Currently, rust-zram-generator can't build due to a too tight dependency on rust-rust-ini. As there hasn't been any breaking API change in the latest version of this crate, we can relax this dependency in order to fix the build.

Moreover, the following changes are made to the packaging:

• we currently patch the source code to avoid relying on a specific env var; this is overkill as we need a custom debian/rules anyway and already pull the needed build dependencies. Drop this patch and export the corresponding env var from the rules file.
• the example config file is installed as the default system-wide configuration, which is inappropriate: this file is meant as a reference sample and doesn't provide the most usable functionality; as a consequence, install this file under the package's documentation and provide a more minimal and useful default configuration

Closes: #1017650

/cc @BenTheTechGuy