rp-pppoe: add license

pkgs/tools/networking/rp-pppoe/default.nix

···
     sed -i Makefile -e 's@PPPOESERVER_PPPD_OPTIONS=@&$(out)@'
   '';
 
-  meta = {
+  meta = with stdenv.lib; {
     description = "Roaring Penguin Point-to-Point over Ethernet tool";
-    platforms = stdenv.lib.platforms.linux;
+    platforms = platforms.linux;
     homepage = https://www.roaringpenguin.com/products/pppoe;
+    license = licenses.gpl2Plus;
   };
 }