Port of libprotobuf for genode

[irgendwie]

• You can test compiling and linking the lib by executing make test/libports/libprotobuf in the build directory
• repos/poe is a small example, using libprotobuf and precompiled proto files from https://github.com/argos-research/protobuf-messages
• You can run the small example with make run/poe or just compile it by using make poe

@dk87

[irgendwie]

We moved our ports to https://github.com/argos-research/genode-world -> libprotobuf was added by https://github.com/argos-research/genode-world/pull/2