goprotobuf: Required extensions don't make sense.

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://codereview.appspot.com/101370050

protoc-gen-go/testdata/extension_user.proto

@@ -66,7 +66,7 @@ message LoudMessage {
 // Extend inside the scope of another type, using a message.
 message LoginMessage {
   extend extension_base.BaseMessage {
-    required UserMessage user_message = 16;
+    optional UserMessage user_message = 16;
   }
 }