ci: Enable -Werror

We don't want to ignore warnings do we?

.github/workflows/haskell-ci.yml

@@ -136,6 +136,8 @@ jobs:
           echo "package xmonad" >> cabal.project
           echo "    ghc-options: -Werror=missing-methods" >> cabal.project
           cat >> cabal.project <<EOF
+          package xmonad
+            flags: +pedantic
           EOF
           $HCPKG list --simple-output --names-only | perl -ne 'for (split /\s+/) { print "constraints: $_ installed\n" unless /^(xmonad)$/; }' >> cabal.project.local
           cat cabal.project

.github/workflows/stack.yml

@@ -65,4 +65,5 @@ jobs:
           set -ex
           stack test \
             --fast --no-terminal \
-            --resolver=${{ matrix.resolver }} --system-ghc
+            --resolver=${{ matrix.resolver }} --system-ghc \
+            --flag=xmonad:pedantic