automoc: workaround for ranlib warning breaks reproducible builds

In order to have at least one valid symbol, !7558 (merged) added a dummy function to the automoc-generated sources. However, the function name is derived from the full path, which makes the resulting binary content depend on the build location.

I'm not sure if it is a good idea to introduce random symbols into an object file in the first place. For reproducible builds, the hashed path should at least be relative to the project root dir.

Cc @timblechmann