virt-viewer: fix missing schema error

Fixes this error, as seen when trying to open a guest VM when
virt-viewer is accessed over ssh with X forwarding:

  GLib-GIO-ERROR **: Settings schema 'org.gnome.system.proxy' is not installed

A similar issue was fixed for virt-manager in commit
fb8a2b3be7f86481904636721f7fec99c30df389 ("virt-manager: fix missing
schema error")