Fix path to FindFFTW.cmake when using KokkosFFT as subdirectory #178

Job	Run time
lint	11s
check_docker_files / check_docker_files	4s
build_base	0s
build (openmp, gcc, gcc, g++, -DKokkos_ENABLE_OPENMP=ON, native)	4m 52s
build (threads, gcc, gcc, g++, -DKokkos_ENABLE_THREADS=ON, native)	6m 5s
build (serial, gcc, gcc, g++, -DKokkos_ENABLE_SERIAL=ON, native)	4m 44s
build (cuda, nvcc, gcc, g++, -DKokkos_ENABLE_CUDA=ON -DKokkos_ARCH_AMPERE80=ON, native)	12m 22s
build (cuda, nvcc, gcc, g++, -DKokkos_ENABLE_CUDA=ON -DKokkos_ARCH_AMPERE80=ON, host_device, -DKo...	13m 22s
build (hip, rocm, hipcc, hipcc, -DKokkos_ENABLE_HIP=ON -DKokkos_ARCH_VEGA90A=ON, -DKokkosFFT_ENAB...	11m 22s
build (hip, rocm, hipcc, hipcc, -DKokkos_ENABLE_HIP=ON -DKokkos_ARCH_VEGA90A=ON, -DKokkosFFT_ENAB...	12m 21s
build (rocm, rocm, hipcc, hipcc, -DKokkos_ENABLE_HIP=ON -DKokkos_ARCH_VEGA90A=ON, native)	11m 25s
build (rocm, rocm, hipcc, hipcc, -DKokkos_ENABLE_HIP=ON -DKokkos_ARCH_VEGA90A=ON, host_device, -D...	11m 22s
build (sycl, intel, icx, icpx, -DKokkos_ENABLE_SYCL=ON -DKokkos_ARCH_INTEL_GEN=ON, native)	18m 27s
build (sycl, intel, icx, icpx, -DKokkos_ENABLE_SYCL=ON -DKokkos_ARCH_INTEL_GEN=ON, host_device, -...	18m 44s
test (cuda, nvcc, self-hosted, cuda)	9m 20s
test (openmp, gcc, ubuntu-latest)	3m 50s
test (threads, gcc, ubuntu-latest)	4m 2s
test (serial, gcc, ubuntu-latest)	3m 58s
	2h 26m 31s

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