net: fMsgProcWake use LOCK instead of lock_guard

src/net.h

@@ -451,7 +451,7 @@ private:
     const uint64_t nSeed0, nSeed1;
 
     /** flag for waking the message processor. */
-    bool fMsgProcWake;
+    bool fMsgProcWake GUARDED_BY(mutexMsgProc);
 
     std::condition_variable condMsgProc;
     Mutex mutexMsgProc;