protobuf-c was updated to 1.0.0 in F21. That may affect your package build as that version bumps the soversion. You will need to regenerate the protobuf-c files and recompile the package. Briefly you need to touch all the .proto files and recompile by having protobuf-c-devel as BuildRequires.
Fixed in rawhide, F21 build pending protobuf-c-1.0.x appearance in buildroots.
F21 build done.