bind/objc: don't expose internal class types mapped to Go interface.

For interface binding, we need only protocols.

Change-Id: I377ec9c8de0a3414d48272892b169fb94823e8ff
Reviewed-on: https://go-review.googlesource.com/13541
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/genobjc.go

@@ -74,7 +74,7 @@ func (g *objcGen) genH() error {
 	for _, obj := range g.names {
 		named := obj.Type().(*types.Named)
 		switch named.Underlying().(type) {
-		case *types.Struct, *types.Interface:
+		case *types.Struct:
 			g.Printf("@class %s%s;\n", g.namePrefix, obj.Name())
 		}
 		g.Printf("\n")