CI: Also set warnings as errors in C code

ci/build-windows.bat

@@ -4,6 +4,7 @@ mkdir build
 
 REM build
 cd build
+set CFLAGS=/WX
 set CXXFLAGS=/WX
 cmake -GNinja -DCMAKE_BUILD_TYPE=RelWithDebInfo -DLIBICSNEO_BUILD_TESTS=ON ..
 if %errorlevel% neq 0 exit /b %errorlevel%