Fail on warnings in MSW clang CI build

We want to keep the build warning-free and we already use -Werror for
the Unix CI builds, so do it here too, for consistency.

.github/workflows/ci_msw.yml

@@ -151,6 +151,7 @@ jobs:
           cmake -G "MinGW Makefiles" \
                 -DCMAKE_C_COMPILER=clang.exe \
                 -DCMAKE_CXX_COMPILER=clang++.exe \
+                -DCMAKE_CXX_FLAGS=-Werror \
                 -DCMAKE_BUILD_TYPE=Release \
                 -DwxBUILD_SAMPLES=ALL \
                 -DwxBUILD_TESTS=ALL \