cmake: explicitly enable int-conversion warnings While GCC enables int-conversion warnings by default, it will currently only warn about such errors even in case where "-DENABLE_WERROR=ON" has been passed to CMake. Explicitly enable int-conversion warnings by using our `ENABLE_WARNINGS` macro, which will automatically use "-Werror=int-conversions" in case it has been requested by the user. (cherry picked from commit aa0ae59a2a31dc0ee5cc987066903d135a5f9e79)