There is no reason why VNC should always be enabled and not be set to
the default value. We already switched the setting in the "configure"
script in commit 3a6a1256 ("configure: Allow vnc to get disabled with
--without-default-features"), so let's do that in meson_options.txt now,
too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210903081358.956267-3-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>