`IntelFFTW` should be included in here too

It was left out when this was needed during the hackathon, because @ocaisa was tapping into the Intel compilers/MKL and MPICH of his system, rather than installations provided via EasyBuild.

It wasn't clear whether the Intel FFTW wrappers were there, and we didn't want to lose time on that. @ocaisa is already looking into whether the FFTW wrapper libs are there, and how to obtain them if they're not there, so `impmkl` really should include them (just like they are in `iomkl`).