Supply --enable-ssh-agent explicitly
In the upstream gnome-keyring, ssh-agent will be disabled by default
in the next release[1]. It would be future proof to supply
--enable-ssh-agent after that, when building with --with ssh_agent.
1. https://gitlab.gnome.org/GNOME/gnome-keyring/-/merge_requests/60
Signed-off-by: Daiki Ueno <dueno@redhat.com>