[protobuf-c] Build failure due to missing dependency

[renatahodovan]

The build of protobuf-c has been failing since last November due to a missing dependency that Docker is unable to clone:

https://github.com/guidovranken/protobuf-c-fuzzers.git

@guidovranken, it looks like this was originally your repository. Could you confirm whether it has been permanently removed, or if it is (or will be) available elsewhere? Thanks in advance!