pd-lua dynamic linking woes on Debian/Ubuntu

As @Joseph reported, at least on Linux Mint 20 pd-lua is broken (doesn't load) OOTB because liblua.so.5.3 is missing. (This doesn't seem to happen on a stock Ubuntu install, apparently by sheer luck it has this library in the default installation, or as a dependency of some package in the default install.)

This dependency isn't auto-detected for some reason when building Debian/Ubuntu packages using the debuild stuff (e.g., on the OBS), so it needs to be added explicitly to the runtime dependencies of the package.