digital: ofdm benchmark_rx fft conflict fix

Fixes the following error in the gr 3.7 OFDM benchmark_rx code due to conflict with numpy 'fft' and gnuradio's 'fft':

Traceback (most recent call last):
  File "./benchmark_rx.py", line 122, in <module>
    main()
  File "./benchmark_rx.py", line 111, in main
    tb = my_top_block(rx_callback, options)
  File "./benchmark_rx.py", line 55, in __init__
    self.rxpath = receive_path(callback, options)
  File "/home/m/play/gr/pybombs/src/gnuradio/gr-digital/examples/ofdm/receive_path.py", line 50, in __init__
    callback=self._rx_callback)
  File "/usr/local/lib/python2.7/dist-packages/gnuradio/digital/ofdm.py", line 224, in __init__
    options.log)
  File "/usr/local/lib/python2.7/dist-packages/gnuradio/digital/ofdm_receiver.py", line 128, in __init__
    self.fft_demod = fft.fft_vcc(fft_length, True, win, True)
AttributeError: 'module' object has no attribute 'fft_vcc'