kernel: Add headerTip method to notifications

This commit is part of the libbitcoinkernel project and seeks to remove
the ChainstateManager's and, more generally, the kernel library's
dependency on interface_ui with options methods in this and the following
few commits. By removing interface_ui from the kernel library, its
dependency on boost is reduced to just boost::multi_index.

src/node/kernel_notifications.cpp

@@ -13,4 +13,9 @@ void KernelNotifications::blockTip(SynchronizationState state, CBlockIndex& inde
     uiInterface.NotifyBlockTip(state, &index);
 }
 
+void KernelNotifications::headerTip(SynchronizationState state, int64_t height, int64_t timestamp, bool presync)
+{
+    uiInterface.NotifyHeaderTip(state, height, timestamp, presync);
+}
+
 } // namespace node