chore(frontends/lean/builtin_exprs): fix compilation warning
chore(frontends/lean/builtin_exprs): fix compilation warning

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