Draft: Add pkg-config dependency

We need pkg-config to build Workhorse components that are integrated via cgo, the C/Golang native bridge. It uses pkg-config to determine which cflags and ldflags need to be set when compiling and linking native libraries.

I discovered the problem via omnibus-gitlab!4560 (closed) when triggering an Omnibus build pipeline downstream: https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/736392740

I'm not sure when all the different Docker images are used. That specific pipeline used the ubuntu:16 image.

This work is required to support gitlab-workhorse!578 (closed) (integrating workhorse with libseccomp).