In #5127 and #5380, @WeiqunZhang updated our AnyFFT wrappers to use MKL FFT calls to ensure we can run spectral solvers on GPUs when compiling with SYCL.

From a recent build in Spack CI (warpx_sycl_rz.txt), it looks like the file Source/FieldSolver/SpectralSolver/SpectralFieldDataRZ.cpp still has direct calls to FFTW in it that show up for RZ+SYCL builds.