Fix the build error

There was an error during the compile:

src/artifact.cpp: In function ‘std::string mk_artifact_id()’:
src/artifact.cpp:100:58: error: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘std::vector<std::basic_string<char> >::size_type {aka long unsigned int}’ [-Werror=format=]
     sprintf(buff,"artifact_%d", artifact_itype_ids.size());