fix(library/vm/vm): compilation error when multi threading is disabled
fix(library/vm/vm): compilation error when multi threading is disabled

rlewis1988/lean,leanprover/lean,digama0/lean,dselsam/lean,avigad/lean,johoelzl/lean,fpvandoorn/lean,leodemoura/lean,avigad/lean,leodemoura/lean,fpvandoorn/lean,digama0/lean,leodemoura/lean,leanprover-community/lean,avigad/lean,fpvandoorn/lean,digama0/lean,johoelzl/lean,avigad/lean,fpvandoorn/lean,leanprover-community/lean,leanprover-community/lean,rlewis1988/lean,dselsam/lean,leanprover/lean,johoelzl/lean,leanprover/lean,fpvandoorn/lean,digama0/lean,rlewis1988/lean,leodemoura/lean,rlewis1988/lean,Kha/lean,fpvandoorn/lean,leanprover/lean,dselsam/lean,Kha/lean,avigad/lean,leanprover/lean,digama0/lean,leanprover-community/lean,leodemoura/lean,Kha/lean,Kha/lean,johoelzl/lean,digama0/lean,rlewis1988/lean,leanprover-community/lean,Kha/lean,dselsam/lean,Kha/lean,dselsam/lean,avigad/lean,dselsam/lean,leanprover/lean,johoelzl/lean,leodemoura/lean,rlewis1988/lean,leanprover-community/lean,johoelzl/lean