Add Privileges schema to allow list

The allow list should contain all schemas implemented by bmcweb.

The allow list was missing the Privileges schema. The Privileges schema
is referenced in the Role schema here:

"AssignedPrivileges": {
  "description": "The Redfish privileges for this role.",
  "items": {
    "$ref": "http://redfish.dmtf.org/schemas/v1/Privileges.json#/definitions/PrivilegeType"

https://redfish.dmtf.org/schemas/v1/Role.v1_3_1.json

In OpenBMC CI Redfish Validator testing the following error was hit 3
times:

ERROR - ConnectionError on http://redfish.dmtf.org/schemas/v1/Privileges_v1.xml:
ConnectionError(ProtocolError('Connection aborted.',
RemoteDisconnected('Remote end closed connection without response')))

Network glitch somewhere but inspired this change.

Long term not sure if the allow list makes sense. While still used today
(and for the immediate future) it should attempt to accurately reflect
the implemented schemas in bmcweb.

The allow list was added in 349a2ac3 to save space.

Information on the CI Redfish Validator testing can be found at
https://github.com/openbmc/openbmc-test-automation/search?q=Redfish-Service-Validator


The test redownloads the schemas each time.

Tested: Validator passed.
See the new schema:
{
  "@odata.id": "/redfish/v1/JsonSchemas/Privileges"
},

Change-Id: Iaf3de36b211efab4115a17fd43f0263b8bd3107f
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>