Vulkan_FFT Benchmark Targets Always ON

I noticed that the `Vulkan_FFT` target with VkFFT benchmarks always builds alongside ITKVkFFTBackend. While there are scenarios where it is useful to have the benchmarks on hand, the typical ITK user will at best want this to be optional. Should revisit how we are setting and building targets, possibly contributing back to VkFFT upstream to make the target configurable.