debian: Fix the protobuf-c dependency of libriemann-client-dev

Instead of depending on libprotobuf-c0-dev in libriemann-client-dev,
depend on the version we were compiled against. With this stricter
dependency, our own users will not be confused when they have the older
protobuf library installed. On the other hand, figuring out the
dependency at build-time still allows easy backports.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>