Enable use of proxy in updater - SL #4630

As documented here [1] (See Session features), the proxy resolver
is a optional feature and need be added to the session.
Fixes 4630

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>

[1] https://developer.gnome.org/libsoup/stable/libsoup-client-howto.html