NEWS: mention the sd_varlink_field_type_t breakage

NEWS: mention the sd_varlink_field_type_t breakage

Follow-up for 93d768e0f36a62afed7ebbf3abe3385cfd186480. The commit with
the fix didn't mention this, but the reported reproducer was:
> Install openSUSE Tumbleweed with account-utils and systemd v258.
> Compile and install systemd v260. Run "varlinkctl list-methods
> /run/account/newidmapd-socket" -> the newidmap service crashes in
> varlink_idl_format_all_fields(). Recompile newidmap with systemd v260
> headers -> varlinkctl list-methods works again.

Other people might hit the same issue, so let's mention that this was
fixed.