add MITK_USE_CUDA superbuild flag

[jtfcordes]

Fixes errors during package generation due to unneccessary CUDA dependencies.

[kislinsk]

Did you add CUDA as a dependency to your fork? We don't have it but the MITK_USE_<...> variables are kind of reserved and introducing MITK_USE_CUDA just for the sake of explicitly disabling it would make it necessary to add an according exception to the _package_excludes list in CMake/PackageConfigurations/All.cmake.

For example, CMakeExternals/OpenCV.cmake just unconditionally disables CUDA support in OpenCV with -DWITH_CUDA:BOOL=OFF. I guess it would be okay to disable it for Eigen without any additional checks as well, given that you didn't introduce CUDA as dependency in your fork.