linuxPackages.usbip: clarify license

pkgs/os-specific/linux/usbip/default.nix

···
   meta = with lib; {
     homepage = "https://github.com/torvalds/linux/tree/master/tools/usb/usbip";
     description = "allows to pass USB device from server to client over the network";
-    license = licenses.gpl2;
+    license = with licenses; [ gpl2Only gpl2Plus ];
     platforms = platforms.linux;
     broken = kernelOlder "4.10";
   };