Fix NO_PUBKEY Error For The Extras Ubuntu Repository

Strangely, the new Ubuntu Extras repository has been added to the Ubuntu 10.10 Maverick Meerkat sources.list by default, yet the repository key has not been imported so if you're using Ubuntu 10.10, you probably get an error when running "sudo apt-get update":

W: GPG error: http://extras.ubuntu.com maverick Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192

To fix it, run the following command:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3E5C1192

That imports the Extras repository key (make sure port 11371 is not blocked or else it won't work).