libnss-libvirt: Update apt seccomp filter

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=934474

Installing libnss-libvirt and enabling it in nsswitch.conf causes the apt http method to issue getdents and getdents64 syscalls, which are not whitelisted in the method's seccomp filter. This makes apt unusable for the most part, and it breaks unattended-upgrades in particular.

/etc/apt/apt.conf.d/90libnss-libvirt already creates an exception for getdents syscalls. Update that file in order to allow getdents64 syscalls as well.

Please note that this is a minimal fix. A more comprehensive version would allow readdir syscalls too, and in fact this is what apt does internally with some other methods. Opinions whether this is a better approach?