google: Add the OAuth2 scope for CardDAV

Turns out that the scope for accessing CardDAV needs to be specified
in the sources too. Having it only in the APIs Console is not enough.
Otherwise Google rejects access to the CardDAV resources with a 401
"AuthSub challenge".

As far as we can make out this is undocumented. For some context see:
9182fb378614abb2ff0245fe71a753f22eaaa906

Fixes: https://bugzilla.gnome.org/707916