licenses: add sissl11

This is Sun Industry Standard Source License 1.1:
https://opensource.org/license/sissl

It was voluntarily retired. Still, it's used in some projects.
Specifically, openvswitch ships some files under lib/sflow*[ch] under
this license.

This license will be added to both openvswitch and ovn packages (the
latter embeds openvswitch libraries, including sflow).

lib/licenses.nix

@@ -1213,6 +1213,11 @@ lib.mapAttrs mkLicense (
       fullName = "SGML UG SGML Parser Materials license";
     };
 
+    sissl11 = {
+      spdxId = "SISSL";
+      fullName = "Sun Industry Standards Source License 1.1";
+    };
+
     sleepycat = {
       spdxId = "Sleepycat";
       fullName = "Sleepycat License";