Consume VkFFT as a header-only library

[tbirdso]

At the time of writing ITKVkFFTBackend retrieves the VkFFT source tree at config time with FetchContent_MakeAvailable and adds its targets to the ITKVkFFTBackend build. This adds unnecessary benchmark build steps for consuming the header-only library.

ITKVkFFTBackend should be updated to comply with Section 2.1 of the VkFFT API guide for consuming VkFFT via VkFFT.h and setting appropriate compiler definitions.