Fix Java 1.6 compile

src/google/protobuf/compiler/java/java_message_lite.cc

@@ -730,10 +730,10 @@ void ImmutableMessageLiteGenerator::GenerateSerializeOneExtensionRange(
 void ImmutableMessageLiteGenerator::GenerateBuilder(io::Printer* printer) {
   printer->Print(
     "public static Builder newBuilder() {\n"
-    "  return DEFAULT_INSTANCE.createBuilder();\n"
+    "  return (Builder) DEFAULT_INSTANCE.createBuilder();\n"
     "}\n"
     "public static Builder newBuilder($classname$ prototype) {\n"
-    "  return DEFAULT_INSTANCE.createBuilder(prototype);\n"
+    "  return (Builder) DEFAULT_INSTANCE.createBuilder(prototype);\n"
     "}\n"
     "\n",
     "classname", name_resolver_->GetImmutableClassName(descriptor_));