While debugging an unrelated issue, I noticed this in the
/var/log/zypp/history file in a completely new TW installation:

```
# 2021-05-27 12:53:09 sgml-skel-0.7.1-1.11.noarch.rpm installed ok
# Additional rpm output:
# /var/tmp/rpm-tmp.ss1I6k: line 5: sed: command not found
# I/O error : Broken pipe
#
2021-05-27 12:53:09|install|sgml-skel|0.7.1-1.11|noarch||openSUSE-20210522-0|2122ff76185f0cb41900e7183b903534c1bf7a345fb41af99fe5af286d8e87f0|
2021-05-27 12:53:09|install|sg3_utils|1.46-1.1|x86_64||openSUSE-20210522-0|8ee96d933cbaaeab7cb716a43e76c6f679f3f2a10c85f734592745a42a7243c9|
2021-05-27 12:53:09|install|sed|4.8-3.5|x86_64||openSUSE-20210522-0|3e20c62841696a4d1240a565cc2ee48943a93d13fb740c560fd6fe27422398b0|
```

So %post fails to run because sed is not installed yet.

This SR fixes that.

- %post uses sed, so add `Requires(post): sed`.