ovn: add LGPL2.1+ and SISSL

LGPL2.1 is for ovs-bugtool plugins (recently moved to `tools` output).
SSISL for a small part of ovs library.

pkgs/by-name/ov/ovn/package.nix

···
     '';
     homepage = "https://www.ovn.org";
     changelog = "https://github.com/ovn-org/ovn/blob/refs/tags/${finalAttrs.src.tag}/NEWS";
-    license = lib.licenses.asl20;
+    license = with lib.licenses; [
+      asl20
+      lgpl21Plus # bugtool plugins
+      sissl11 # lib/sflow from ovs submodule
+    ];
     maintainers = with lib.maintainers; [
       adamcstephens
       booxter