update Siesta EasyBlock to use serial FFTW

[mboisson]

The only module of Siesta that uses FFTW is STM, and
./Util/STM/ol-stm/Src/Makefile
explicitly mentions:

  19 #
  20 #  If your main Siesta build used an mpi compiler, you might need to
  21 #  define an FC_SERIAL symbol in your top arch.make, to avoid linking
  22 #  in the mpi libraries even if we explicitly undefine MPI below.
  23 #

Using LIBFFT contains -lfftw3_mpi, which will fail to link when compiled by a serial compiler.

fftw should be set to LIBFFT_MT and not LIBFFT

@akesandgren @dithwick

[migueldiascosta]

fwiw, in my case (using foss/2019b) it doesn't fail while linking with gfortran and -lfftw3_mpi, but it does fail while running stm, with undefined symbol: ompi_mpi_char and this PR solves that.

[akesandgren]

LGTM

[akesandgren]

Going in, thanks @mboisson!