chore(kernel/expr): fix cpplint warning
chore(kernel/expr): fix cpplint warning

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

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