gnuradio: enable Darwin building

pkgs/applications/misc/gnuradio/default.nix

···
     '';
     homepage = https://www.gnuradio.org;
     license = licenses.gpl3;
-    platforms = platforms.linux;
+    platforms = platforms.linux ++ platforms.darwin;
     maintainers = with maintainers; [ bjornfor fpletz ];
   };
 }