gnuradio: add missing dependency to python-numpy for python support

Some gnuradio python blocks need python-numpy to be present. Without
python-numpy, gnuradio-python based apps fails, on runtime, with
traceback like:

Traceback (most recent call last):
  File "./top_block.py", line 18, in <module>
    from gnuradio import analog
  File "/usr/lib/python2.7/site-packages/gnuradio/analog/__init__.py", line 35, in <module>
    from am_demod import *
  File "/usr/lib/python2.7/site-packages/gnuradio/analog/am_demod.py", line 22, in <module>
    from gnuradio import gr
  File "/usr/lib/python2.7/site-packages/gnuradio/gr/__init__.py", line 44, in <module>
    from top_block import *
  File "/usr/lib/python2.7/site-packages/gnuradio/gr/top_block.py", line 30, in <module>
    from hier_block2 import hier_block2
  File "/usr/lib/python2.7/site-packages/gnuradio/gr/hier_block2.py", line 26, in <module>
    import pmt
  File "/usr/lib/python2.7/site-packages/pmt/__init__.py", line 58, in <module>
    from pmt_to_python import pmt_to_python as to_python
  File "/usr/lib/python2.7/site-packages/pmt/pmt_to_python.py", line 22, in <module>
    import numpy
ImportError: No module named numpy

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>