fix(kernel/expr): compilation warning
fix(kernel/expr): compilation warning

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

dselsam/lean,Kha/lean,johoelzl/lean,sp3ctum/lean,htzh/lean,soonhokong/lean-osx,soonhokong/lean-osx,c-cube/lean,rlewis1988/lean,johoelzl/lean,leodemoura/lean,leodemoura/lean,Kha/lean,soonhokong/lean,leanprover-community/lean,sp3ctum/lean,htzh/lean,fgdorais/lean,fpvandoorn/lean2,Kha/lean,htzh/lean,soonhokong/lean-windows,sp3ctum/lean,soonhokong/lean,johoelzl/lean,fgdorais/lean,fpvandoorn/lean2,eigengrau/lean,Kha/lean,UlrikBuchholtz/lean,eigengrau/lean,avigad/lean,soonhokong/lean,fpvandoorn/lean,fgdorais/lean,codyroux/lean0.1,soonhokong/travis_test,leodemoura/lean,dselsam/lean,sp3ctum/lean,soonhokong/travis_test,soonhokong/travis_test,codyroux/lean0.1,leanprover-community/lean,avigad/lean,rlewis1988/lean,fgdorais/lean,digama0/lean,soonhokong/lean-windows,javra/lean,fpvandoorn/lean,fpvandoorn/lean,Kha/lean,codyroux/lean0.1,sp3ctum/lean,Kha/lean,eigengrau/lean,leanprover-community/lean,soonhokong/lean-windows,levnach/lean,rlewis1988/lean,leanprover-community/lean,dselsam/lean,fpvandoorn/lean,UlrikBuchholtz/lean,soonhokong/lean,c-cube/lean,johoelzl/lean,fpvandoorn/lean,soonhokong/lean-windows,javra/lean,soonhokong/travis_test,UlrikBuchholtz/lean,digama0/lean,c-cube/lean,leanprover/lean,levnach/lean,c-cube/lean,leanprover/lean,c-cube/lean,fpvandoorn/lean2,htzh/lean,rlewis1988/lean,avigad/lean,leanprover/lean,leanprover/lean,johoelzl/lean,avigad/lean,codyroux/lean0.1,fpvandoorn/lean2,fpvandoorn/lean2,digama0/lean,avigad/lean,avigad/lean,rlewis1988/lean,UlrikBuchholtz/lean,soonhokong/lean,levnach/lean,fgdorais/lean,rlewis1988/lean,leanprover/lean,leodemoura/lean,leanprover-community/lean,digama0/lean,javra/lean,javra/lean,leodemoura/lean,levnach/lean,fpvandoorn/lean,dselsam/lean,UlrikBuchholtz/lean,eigengrau/lean,eigengrau/lean,leanprover/lean,soonhokong/lean-osx,leodemoura/lean,soonhokong/lean-windows,soonhokong/lean-osx,digama0/lean,johoelzl/lean,levnach/lean,soonhokong/lean-osx,javra/lean,leanprover-community/lean,htzh/lean,dselsam/lean,fgdorais/lean,digama0/lean,dselsam/lean