Add missing incognito property to brave-extension manifest

[bsclifton]

Description

The extension manifest for brave-extension should have it's incognito property matching what is set in the browser-laptop (Muon) version

Per the docs, Chromium would spin up a secure process for the instances using this extension in private mode, which is what we want. Those private instances do not have access to non-private profiles

This change should help in solving #1198

Additional details

You can see the browser-laptop version of the manifest here:
https://github.com/brave/browser-laptop/blob/41e46f9ce7e0d4f7a558646f377e40d29d0e101d/app/extensions.js#L36-L216