Use libiconv in gettext

If libiconv is not linked to gettext, I get the following error during
the compilation of gcc-10.5 (in a system with minimal requirements):

```
INFO msgfmt -o de.mo ../../.././libstdc++-v3/po/de.po
INFO msgfmt: Cannot convert from "ISO-8859-1" to "UTF-8". msgfmt relies on iconv(). This version was built without iconv().
```

See merge request nexedi/slapos!1732

component/gettext/buildout.cfg

@@ -2,6 +2,7 @@
 parts =
   gettext
 extends =
+  ../libiconv/buildout.cfg
   ../libxml2/buildout.cfg
   ../lunzip/buildout.cfg
   ../ncurses/buildout.cfg
@@ -22,6 +23,7 @@ configure-options =
   --disable-more-warnings
   --with-libncurses-prefix=${ncurses:location}
   --with-libxml2-prefix=${libxml2:location}
+  --with-libiconv-prefix=${libiconv:location}
   --with-included-gettext
   --without-emacs
   --disable-acl