builderparser: fix <lookup/> with interface types

builderparser: fix <lookup/> with interface types

If we have a <lookup name="foo" type="SomeInterface"> a runtime warning
would be emitted and the expression would fail to be created. This is
because the interfaces will likely be a GObject as well, meaning we check
the object type branch instead of the interface.

Instead, we need to use the fundamental type like other parts of the
expression system use.