bugfix: Mark m_tip_block_cv as guarded by m_tip_block_mutex

This is not strictly required, but all places using m_tip_block_cv
(except shutdown) already take the lock. The annotation makes it easier
to catch potential deadlocks before review.

Adding the missing lock to the shutdown sequence is a bugfix.

An alternative would be to take the lock and release it before
notifying, see
https://github.com/bitcoin/bitcoin/pull/30967#discussion_r1778899716

src/node/kernel_notifications.h

@@ -58,7 +58,7 @@ public:
     bool m_shutdown_on_fatal_error{true};
 
     Mutex m_tip_block_mutex;
-    std::condition_variable m_tip_block_cv;
+    std::condition_variable m_tip_block_cv GUARDED_BY(m_tip_block_mutex);
     //! The block for which the last blockTip notification was received for.
     uint256 m_tip_block GUARDED_BY(m_tip_block_mutex);