fix(frontends/lean/structure_cmd): compilation warning
fix(frontends/lean/structure_cmd): compilation warning

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