x11-wm/blackbox: require >=gettext-0.20

x11-wm/blackbox: require >=gettext-0.20

On gettext-0.19 autopoint fails as:

```
  ***** PWD: .../blackboxwm-0.75
  ***** autopoint --force

  autopoint: *** The AM_GNU_GETTEXT_VERSION declaration in your configure.ac
                 file requires the infrastructure from gettext-0.20 but this
                 version is older. Please upgrade to gettext-0.20 or newer.
  autopoint: *** Stop.
```

Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Ben Kohler <bkohler@gentoo.org>