Specify -Werror for Stack CI too
Specify -Werror for Stack CI too

Follows cabal builds from 451484509b6dd2934371612629b526b8ef35bd3c until
now.

juhp/stack,martin-kolinek/stack,mrkkrp/stack,anton-dessiatov/stack,deech/stack,AndreasPK/stack,Fuuzetsu/stack,juhp/stack,AndrewRademacher/stack,martin-kolinek/stack,AndrewRademacher/stack,mrkkrp/stack,Blaisorblade/stack,MichielDerhaeg/stack,deech/stack,MichielDerhaeg/stack,anton-dessiatov/stack,AndreasPK/stack,Fuuzetsu/stack,Blaisorblade/stack