fix(util/stackinfo): compilation warning on cygwin/mingw
fix(util/stackinfo): compilation warning on cygwin/mingw

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

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