Merge pull request #1444 from mbrtargeting/master

Added serialVersionUID to ExtendableMessage.

java/core/src/main/java/com/google/protobuf/GeneratedMessage.java

@@ -795,6 +795,8 @@ public abstract class GeneratedMessage extends AbstractMessage
       extends GeneratedMessage
       implements ExtendableMessageOrBuilder<MessageType> {
 
+    private static final long serialVersionUID = 1L;
+
     private final FieldSet<FieldDescriptor> extensions;
 
     protected ExtendableMessage() {