hackrf: allow on darwin

pkgs/applications/misc/hackrf/default.nix

···
     description = "An open source SDR platform";
     homepage = http://greatscottgadgets.com/hackrf/;
     license = licenses.gpl2;
-    platforms = platforms.linux;
+    platforms = platforms.all;
     maintainers = with maintainers; [ sjmackenzie the-kenny ];
   };
 }