Remove requirement for hipFFT

[Peter Eastman]

I'm trying to build using the HIP libraries from conda-forge. There aren't currently any libraries for hipFFT, which leads CMake to produce an error. It shouldn't actually be needed, since the default is to use vkFFT, but CMake still refuses to continue if it isn't present. Could that requirement be removed?

[Anton Gorenko]

I have pushed a commit with optional hipFFT to this PR: #14