libgimp: fix more gi-docgen "qualifier fragment".

Now the warning is:

WARNING: Invalid fragment for 'Gimp.Config': it should be struct
It implements the [iface@Config] interface and therefore has all its
                  ^~~~~~~~~~~~~~

This warning feels wrong as we use GimpConfig for the name of the
interface, yet in the .gir file, I see `<record name="Config" …>` yet
`<interface name="ConfigInterface" …>`.
I suppose gi-docgen would want us to use [iface@ConfigInterface] if we
want to link the interface. But looking at the gir file, all interesting
interface methods are associated to the Config record. So let's just go
with the [struct@Config] proposed by the warning.
In any case, something feels wrong or broken here, but we need to fix
this for the CI. I hope Niels can look at it at some point.

libgimp/gimpprocedureconfig.c

@@ -39,7 +39,7 @@
  * procedure's arguments and auxiliary arguments in number, order and
  * type.
  *
- * It implements the [iface@Config] interface and therefore has all its
+ * It implements the [struct@Config] interface and therefore has all its
  * serialization and deserialization features.
  *
  * Since: 3.0