elements/cups-base.bst: ship lpr binary in flatpak runtimes

Some apps use lpr binary instead of talking to cups through socket. Moreover java hardcodes path to /usr/bin which makes bundling lpr by apps ineffective.

This solves the problem by making lpr part of cups-libs which is part of flatpak runtimes.

Fixes #1636 (closed)

Fixes #1048 (closed)