gpio-utils: add license

pkgs/os-specific/linux/kernel/gpio-utils.nix

···
 
   meta = {
     description = "Linux tools to inspect the gpiochip interface";
-    maintainers = with stdenv.lib.maintainers; [ kwohlfahrt ];
-    platforms = stdenv.lib.platforms.linux;
+    maintainers = with maintainers; [ kwohlfahrt ];
+    platforms = platforms.linux;
+    license = licenses.gpl2;
   };
 }