fix(config): keep protobuf Message outside namespace ripple

Forward-declaring google inside ripple made google::protobuf resolve to
ripple::google::protobuf and broke overlay ZeroCopyStream.

src/xrpld/core/Config.h

@@ -45,17 +45,17 @@
 #include <utility>
 #include <vector>
 
-namespace ripple {
-
-class Rules;
-class Message;
-
 namespace google {
 namespace protobuf {
 class Message;
 }
 }  // namespace google
 
+namespace ripple {
+
+class Rules;
+class Message;
+
 //------------------------------------------------------------------------------
 
 enum class SizedItem : std::size_t {