net_processing: add thread safety annotation for m_highest_fast_announce

Author: ajtowns

src/net_processing.cpp

@@ -863,7 +863,7 @@ class PeerManagerImpl final : public PeerManager
     std::atomic_bool m_headers_presync_should_signal{false};
 
     /** Height of the highest block announced using BIP 152 high-bandwidth mode. */
-    int m_highest_fast_announce{0};
+    int m_highest_fast_announce GUARDED_BY(::cs_main){0};
 
     /** Have we requested this block from a peer */
     bool IsBlockRequested(const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs_main);