cue: observe JSON mappings for google.protobuf.Struct

Change-Id: I234767c9bcdb67dee3492827cf598f5a45258959
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2681
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/protobuf/parse.go b/encoding/protobuf/parse.go
index b3e7451..48617c9 100644
--- a/encoding/protobuf/parse.go
+++ b/encoding/protobuf/parse.go
@@ -330,7 +330,9 @@
 		return err
 	}
 
-	p.mapBuiltinPackage(v.Position, v.Filename, filename == "")
+	if !p.mapBuiltinPackage(v.Position, v.Filename, filename == "") {
+		return nil
+	}
 
 	imp, err := p.state.parse(filename, nil)
 	if err != nil {