apt-cacher-ng: allow read access to /proc/sys/kernel/random/uuid

As reported by Paul Wise pabs@debian.org, apt-cacher-ng 3.6 started using the asynchronous libevent DNS resolver, which implies reading /proc/sys/kernel/random/uuid.

apt-cacher-ng starts anyway without this permission, but I'm wary of the consequences of not being able to read that random UUID: since this is network-related, it could for example imply using more predictable identifiers in generated packets, which could be a security problem.

Bug-Debian: https://bugs.debian.org/983006