swig: add meta.platforms (set to linux)

So that Hydra will build swig. This provides binary packages to users
and build breakage detection.
gnuradio: wrap example programs too

Without this the examples don't work. Example:

$ /nix/store/HASH-gnuradio-3.7.1/share/gnuradio/examples/uhd/usrp_am_mw_rcv.py
Traceback (most recent call last):
  File "/nix/store/HASH-gnuradio-3.7.1/share/gnuradio/examples/uhd/usrp_am_mw_rcv.py", line 24, in <module>
    from gnuradio import gr, eng_notation
ImportError: No module named gnuradio
gnuradio: update 3.7.1 -> 3.7.2.1

I verified that gnuradio-companion (the GUI) still starts.