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).

Changed files
+5
lib
+5
lib/licenses.nix
···
fullName = "SGML UG SGML Parser Materials license";
};
sleepycat = {
spdxId = "Sleepycat";
fullName = "Sleepycat License";
···
fullName = "SGML UG SGML Parser Materials license";
};
+
sissl11 = {
+
spdxId = "SISSL";
+
fullName = "Sun Industry Standards Source License 1.1";
+
};
+
sleepycat = {
spdxId = "Sleepycat";
fullName = "Sleepycat License";