host_bios_upgrade: Add Updateable Association

Let BIOS Object make updateable association, so that
Redfish would show "Updateable": true just like other

Testing:
curl https://*/UpdateService/FirmwareInventory/bios_active
{
  "@odata.id": "/*/UpdateService/FirmwareInventory/bios_active",
  "@odata.type": "#SoftwareInventory.v1_1_0.SoftwareInventory",
  "Description": "Host image",
  "Id": "bios_active",
  "Name": "Software Inventory",
  "RelatedItem": [
    {
      "@odata.id": "/redfish/v1/Systems/system/Bios"
    }
  ],
  "RelatedItem@odata.count": 1,
  "Status": {
    "Health": "OK",
    "HealthRollup": "OK",
    "State": "Enabled"
  },
  "Updateable": true,
  "Version": "null"
}

Change-Id: I22614f7e5bf55764c785ad6c085162c5a3a45cc2
Signed-off-by: Daniel Hsu <Daniel-Hsu@quantatw.com>
diff --git a/item_updater.cpp b/item_updater.cpp
index 9cb9467..9bbd6c8 100644
--- a/item_updater.cpp
+++ b/item_updater.cpp
@@ -859,6 +859,7 @@
 
     createActiveAssociation(path);
     createFunctionalAssociation(path);
+    createUpdateableAssociation(path);
 
     auto versionId = path.substr(pos + 1);
     auto version = "null";