component/re6stnet: correctly extend stack/slapos.cfg

Without this fix, we cannot compile re6st-node because we don't use the
correct version of setuptools.

component/re6stnet/buildout.cfg

 [buildout]
 extends =
+  ../../stack/slapos.cfg
   ../defaults.cfg
   ../babeld/buildout.cfg
   ../geoip2/buildout.cfg