rfnoc_modtool: Fix missing 'out' line in data outputs section

When using the rfnoc_modtool YAML creator wizard, there should have been
a line `out:` generated in the `outputs` section, which is now added.

host/python/uhd/rfnoc_utils/templates/blocktool/descriptor.yml.mako

@@ -96,6 +96,7 @@ data:
   outputs:
     # This will generate one or more ports all called 'out'. It is also
     # possible to have multiple ports with different names.
+    out:
 %    if data_iface_type == 'axis_data':
       num_ports: ${ num_outputs }
       item_width: ${ out_item_width }