chore(frontends/lean/print_cmd): compilation error on g++ 8.2
chore(frontends/lean/print_cmd): compilation error on g++ 8.2

dselsam/lean,fpvandoorn/lean,soonhokong/lean-windows,dselsam/lean,leodemoura/lean,leanprover-community/lean,fpvandoorn/lean,rlewis1988/lean,digama0/lean,leanprover-community/lean,fgdorais/lean,leodemoura/lean,leanprover-community/lean,leanprover/lean,fgdorais/lean,leodemoura/lean,johoelzl/lean,digama0/lean,soonhokong/lean-osx,leanprover/lean,soonhokong/lean-osx,Kha/lean,Kha/lean,leanprover-community/lean,leodemoura/lean,johoelzl/lean,rlewis1988/lean,leanprover-community/lean,Kha/lean,avigad/lean,digama0/lean,Kha/lean,soonhokong/lean-osx,leanprover/lean,leanprover/lean,johoelzl/lean,fgdorais/lean,johoelzl/lean,digama0/lean,johoelzl/lean,dselsam/lean,dselsam/lean,dselsam/lean,Kha/lean,leanprover-community/lean,johoelzl/lean,rlewis1988/lean,fpvandoorn/lean,soonhokong/lean-osx,soonhokong/lean-windows,avigad/lean,rlewis1988/lean,rlewis1988/lean,leodemoura/lean,soonhokong/lean-windows,fpvandoorn/lean,digama0/lean,soonhokong/lean-windows,leanprover/lean,avigad/lean,fgdorais/lean,avigad/lean,avigad/lean,leodemoura/lean,soonhokong/lean-osx,avigad/lean,soonhokong/lean-windows,rlewis1988/lean,fpvandoorn/lean,fgdorais/lean,fgdorais/lean,fpvandoorn/lean,digama0/lean,dselsam/lean,leanprover/lean,Kha/lean