Fix cygwin compilation error
Fix cygwin compilation error

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

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