'delete' compiler warnings from FFT.cc

Compiling FFT.cc on macOS with clang-13 emits these warnings

/Users/duncanmacleod/opt/mambaforge/conda-bld/gwollum_1652953354999/work/src/Utils/FFT.cc:86:18: warning: 'delete' applied to a pointer-to-array type 'fftw_complex *' (aka 'double (*)[2]') treated as 'delete[]'
  if(xc_t!=NULL) delete xc_t;
                 ^      ~~~~
                       []
/Users/duncanmacleod/opt/mambaforge/conda-bld/gwollum_1652953354999/work/src/Utils/FFT.cc:87:3: warning: 'delete' applied to a pointer-to-array type 'fftw_complex *' (aka 'double (*)[2]') treated as 'delete[]'
  delete x_f;
  ^      ~~~
        []
2 warnings generated.