Move ChainstateManagerOpts into kernel:: namespace

It should have been there in the first place.

src/kernel/chainstatemanager_opts.h

@@ -10,6 +10,8 @@
 
 class CChainParams;
 
+namespace kernel {
+
 /**
  * An options struct for `ChainstateManager`, more ergonomically referred to as
  * `ChainstateManager::Options` due to the using-declaration in
@@ -20,4 +22,6 @@ struct ChainstateManagerOpts {
     const std::function<int64_t()> adjusted_time_callback{nullptr};
 };
 
+} // namespace kernel
+
 #endif // BITCOIN_KERNEL_CHAINSTATEMANAGER_OPTS_H