fftwMpi: add attribute for MPI-enabled fftw

pkgs/top-level/all-packages.nix

@@ -18036,6 +18036,7 @@ with pkgs;
   fftwSinglePrec = fftw.override { precision = "single"; };
   fftwFloat = fftwSinglePrec; # the configure option is just an alias
   fftwLongDouble = fftw.override { precision = "long-double"; };
+  fftwMpi = fftw.override { enableMpi = true; };
 
   filter-audio = callPackage ../development/libraries/filter-audio {};