Add missing HTTP_X_FORWARDED_FOR

See discussion [here](#7876) for background.

From the [doc](https://netbox.readthedocs.io/en/stable/customization/custom-scripts/) i should be able to access `META.HTTP_X_FORWARDED_FOR` but i was not able to since they were not being sent downstream

Author: davama

netbox/utilities/constants.py

@@ -57,6 +57,7 @@
     'HTTP_HOST',
     'HTTP_REFERER',
     'HTTP_USER_AGENT',
+    'HTTP_X_FORWARDED_FOR',
     'QUERY_STRING',
     'REMOTE_ADDR',
     'REMOTE_HOST',